Space program historian here. During the Apollo 1/AS-204 "plugs out test" fire, there was a set protocol in place for emergency egress from the capsule while it was on the launch pad. The basics of the procedures were for the senior pilot (in this case, Ed White), seated in the center couch position, to reach over his head and unlatch and remove the inner hatch while keeping his harness buckled. His body was found with his harness still buckled, and reports of witnesses and camera footage showing the view inside the cockpit through the small hatch window stated that it appeared White was indeed reaching over his head to attempt to open the hatch as fire swept through the spacecraft. Gus Grissom, seated to the left of White, was expected to open the cabin relief dump valves at the beginning of the emergency egress procedure (the controls for which were located on the control panel to the left of Grissom's couch), and that action had apparently been at least partially accomplished according to the cockpit control settings discovered in the post-accident cockpit controls settings analysis. In fact, the cabin relief dump valve control knobs had apparently been bent from being struck with considerable force - presumably by Grissom, since he was the only one who could reach those controls, but to my knowledge it was not ever actually confirmed whether the cabin relief dump valve control was positively activated. We do know that the hull of the capsule actually ruptured from the overpressurization after the fire started though. The emergency egress checklist also called for Gus Grissom to lower Ed White's headrest to give White easier access to the inner hatch release handle ratcheting mechanism that needed to be used to open the hatch during an emergency egress. Post accident investigation revealed that White's headrest had been lowered as required, leading one to assume Grissom had done so as he was supposed to. It is believed that Grissom was also struggling to aid White in trying to ratchet the hatch open (determined by the placement of Grissom's body and his hands when the capsule was finally cracked open from the outside by launch pad personnel). There were other steps as well, involving disconnecting their life support hosing from the CM's environmental control system after getting the hatch open but prior to egress, and unbuckling their restraint harnesses, but they never got that far before being overwhelmed by noxious gases from the fire.

Unfortunately, it was expected to take the crew roughly 60 seconds to carry out an unaided (with no help from the outside of the capsule) emergency egress, due to the hatch design. In that time, the fire overwhelmed them and caused the capsule to dramatically overpressurize even further, making it essentially impossible to open the inward-opening hatch the Block 1 AS-204 capsule was fitted with. Their spacesuits ruptured from the heat and their O2 hoses were compromised, and it is believed they died of carbon monoxide exposure.

There is a brief NASA document titled "Apollo Egress Criteria", which I believe was written by a NASA team known as either the "Hazardous Emergency Egress Working Group" or the "Apollo Egress Working Group". If memory serves, this document is from the early 1960s, and focuses on the various egress (both standard and emergency) procedures for both the Apollo Block 1 CSM and LM designs, as well as discussing the checklists for emergency pad egress in case of fire or serious booster malfunction. I am pretty sure I have a hard copy of this document at home as I have read it before, and when I go home in a couple days I will try to dig my copy up for you. I cannot seem to find a copy of the specific document online right now via a quick Googling for it, though I do see there are references to it mentioned on various NASA sites and in various documents related to the Apollo 1/AS-204 fire. I will keep digging around and see if I can pull up the actual Apollo CSM Block 1 AS-204 emergency pad egress checklist online somewhere, as I know it exists. In fact, the crew was supposed to finish their "plugs out" test that day by conducting an emergency egress drill, which they had trained for in the aptly named "capsule egress trainer" simulator located at KSC.