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:

1. Click on the More icon (3 bars next to More at the bottom)
   1. A drop-down menu appears
2. Click on Edit Message
   NOTE: Edits will be emailed to group members. Please be considerate in the number of edits you make.
   • One way to avoid editing is to Preview your message.
3. (Optional) - Explain why you edited the message
4. 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.