I would like to know if there is a way to rename an Area 51 proposal group? I didn't find anything related anywhere.
Anyone with 1k rep can edit proposals in definition phase, which includes renaming them. You can edit your own proposals without 1k rep. The edit link is right under the proposal description. Once a proposal gets to commitment phase, edits are limited to ♦ moderators (here just SE employees).
If the name change you're making/proposing is at all controversial, it's not a bad idea to start a discussion first about what the best name is.