For computer:
1. Go to your inbox
2. Open the chat you wish to change the name of
3. Scroll over the title
4. You will see a plus sign and a pencil (it may be hidden behind people's names)
5. Click the pencil
6. This will open a bubble containing a space to edit the title, and a chance to upload a conversation picture
7. Erase the title currently highlighted in blue, in the edit title section.
8. Create your own title
9. Click save
10. You now have a new name for your group ;)