How do I delete a Group?

To delete or leave a group:

Go to Settings | Groups & Contacts and tap Edit next to the group. You can also open the group from the Contacts tab and tap its name at the top. Either way, you land on Group Info.
Tap the 3-dot menu at the top right.
Choose what you want to do:
- Delete group removes the group for all of its members, so nobody can send messages in it anymore.
- Leave group takes only you out of the group, and you stop receiving its messages.

Settings → Groups & Contacts → Edit → 3-dot menu → Delete group

Want to keep the group and just change it? Edit Group Info in the same menu lets you rename it and change its photo. See Naming a Group and Setting a Photo.