Marvel hasn't had an "official" reboot. However, over the years they updated or changed continuity or story aspects of individual titles to handle the fact that the Marvel Universe has been around since (our time) 1960 or thereabouts.
Some examples include:
- Reed and Ben were originally WWII vets. Now I'm not sure if either of them were ever in the military.
Tony Stark was originally injured in Vietnam. Now, [I think] he was injured in Afghanistan.
Captain America was in the ice waaaay longer than he used to be.
In an early Spiderman story, Peter went on the Ed Sullivan show. Obviously, in current Marvel continuity, that couldn't have happened.
In the case of Squadron Supreme, I'm not sure there's ever been a reboot. Since the Squadron was always depicted as coming from a parallel earth I've just assumed the Squardon being written about these days is simply another parallel universe's version of the group.
