See also the official documentation:
Help Center > Working with group messages > Editing your own messages in the group's archive
Depending on the settings for your Group (set by Owners and/or Moderators) you may or may not be allowed to edit your own messages after you've posted them.
If allowed, here is how to do it:
- Click on the More icon (3 bars next to More at the bottom)
- A drop-down menu appears
- Click on Edit Message
NOTE: Edits will be emailed to group members. Please be considerate in the number of edits you make.
- (Optional) - Explain why you edited the message
- Click Save And Send To Group
NOTE: Depending on your Group's settings, your message may be delayed until it is approved by a Moderator.