As the advertisement says, just do it.  You can’t change the original paperwork. That is an historic fact.  But you can cease to be the current president and resign as an officer, director or active participant in any other way.  All you have to do is submit a letter saying you resign, and you can move on with no further responsibility.

When you are no longer president, they will have to remove that designation after your name on the letterhead and any other materials.  If you don’t want to continue to be listed as “founder,” you can ask them to remove your name altogether.  But if it is still a good organization, the identification may be a nice thing to have. You should be proud that you helped build an organization that doesn’t require conflict to assure transition from the founders to a board that is “going strong.”