Why did Hitler change the name of his party to the "National Socialist German Workers Part (N.S.D.A.P.) from the "German workers party" If it's quite clear he was far right and had a hatred for Marx and his ideas?

When Anton Drexler founded the German Workers Party in Munich he was most certainly aware of the German Workers Party in Austria founded in 1903. In August 1918 they had changed their name in German National Socialist Workers Party (DNSAP). Their main opponent through the times before had been the Social Democratic Workers Party (SDAP). The rebranding was an attempt to attract their voters. The claim was that the Social Democrats by promoting Marxism and the unification of the working class of all nations in Austria betrayed the German people (internal socialism). Instead national socialism proclaimed to unite the German business owners and the German workers against the unproductive Jews and Slavic peoples. They were to blame for low wages, high interest rates and dire social conditions.

At the end of 1919 the Austrian got in touch with in the Munich group. Hitler most certainly embraced the claim of the Austrians that they were not just another conservative German nationalistic group but the bearer of a new ideology unifying the German people under one banner.

see:

Whiteside, Andrew Gladding (1962): Austrian national socialism before 1918. The Hague: M. Nijhoff.

Bruegel, J.W (1979): Nazis without Hitler? Nazis before Hitler! The Story of the DNSAP. In: East Central Europe 6, p. 46. DOI: 10.1163/187633079X00033.