Is there any instances of a coup being a good thing or bringing proper democracy
I want to be cautious about referring to any coup as "good." Whether or not any non-peaceful change of regime is "good" is a question too difficult a question to answer with a one-size-fits-all response. It raises corollary questions of constitutionalism - how could a coup be "good" if it violates the codified precepts binding the political community of the state? - as well as utilitarian ethics - conversely, how would it be "good" for persons of influence, such as senior politicians or military commanders, to allow a tyrannical regime to enact a reign of terror?
Reaching far back into history for an example of the above, let's look at the coup through which Zhao Kuangyin, a general in the tenth century Chinese state of Later Zhou, came to power as the first emperor of the well-known Song dynasty. By 960 CE, Later Zhou was the latest in a series of states to rule large portions of modern China from the city of Bianjing (modern Kaifeng). Its founder had, just a decade previously, overthrown the tyrannical emperor of the Later Han, a dynasty that had endured for only four years. The consequence of all this regime change, quite understandably, was political uncertainty, a point that Peter Lorge illustrates very well in The Reunification of China: Peace Through War Under the Song Dynasty.
Back to Zhao Kuangyin - he'd risen to relative prominence under Emperor Shizong of Later Zhou, who had acted decisively to shore up his political power, return stability to the state, and expand the empire's territory. But he died young, leaving a young heir in charge - a recipe for disaster, if you're familiar with this period of Chinese history, when minor-aged emperors were routinely deposed by folks who didn't last long, you'll know that this scenario could have easily led to bloody rebellions, civil wars, and nasty power struggles that could have made the state susceptible to foreign invasion and conquest (and there were plenty of territorially avaricious neighbors to keep at bay). So Zhao, figuring himself as capable of anyone at maintaining the state created by Shizong, mounted a near-bloodless coup that sent the young emperor into comfortable retirement and placed himself on the throne in his stead. Zhao kept most of the Northern Zhou bureaucracy and ministerial leadership intact and set a strong precedent for both military competence and benevolent domestic rule, initiating the reunification of China and ushering in a very prosperous and (for the era) liberal regime.
Was this a "good" coup? You might say so, given the potential cataclysms that Zhao's seizure of power and deft politicking took off the table. But then again, we're looking at this retrospectively. There was undoubtedly some brutal violence meted out over the course of the regime change - one of Northern Zhou's loyalist generals was murdered, and an opportunistic Song partisan later murdered the retired boy emperor without authorization from Zhao Kuangyin. Ethically, such actions are unjustifiable. And more broadly, did the common folk of Northern Zhou benefit directly from the seizure of power by a savvier ruler? Did their quality of life change significantly from 959 to 960 CE? These are all questions that would need to be considered in detail to assess the "goodness" of this particular coup, and as you can imagine, similar questions about context, character, and social ramifications would have to be considered for any similar seizure of power.
Enough of me carrying on. Insofar as your question pertains to coups that exchanged an authoritarian political regime for a democratic one, then we are discussing one of two generally-recognized forms of coups d'etat, the "regime change coup." These can take many forms, but are distinguished from scenarios in which the structure of the regime remains unchanged despite a switch-up in the leading actors (the mid-1980s removal of Tunisia's Habib Bourguiba by his longtime subordinate Zine el Abidine Ben Ali).
If you're interested in the academic framework behind coup classifications, there's been a lot of fascinating recent work by entities like the Cline Center at the University of Illinois and scholars like John Chen, David Carter, and Joseph Wright, who this year introduced their "Colpus Dataset" enumerating and classifying coups d'etat.
Back to your specific question - if we are looking specifically at authoritarian-to-democratic regime change coups, then frankly, those are not very common. As Chen, Carter, and Wright point out in their introduction to the Colpus Dataset, it's rare that a coup d'etat seeks to replace autocratic rule with democratic rule. Additionally, as I'll mention in an upcoming example, it can sometimes be difficult to identify an authoritarian-to-democratic coup because of the intermediate steps that can be involved (e.g. the creation of a transitional government that does not itself rule democratically, but which brings about a democratic - or at least more democratic - regime).
With that said, there are at least a few examples of coups that resulted, directly or indirectly, in the replacement of authoritarian governments with more democratic ones: