Once a guild has been created, you cannot change its name. The guild leader can edit some parameters.
To do this:
1. Open the Guild window and go to the guild Chat tab
2. Tap the logo of your guild in the top left-hand corner of the window
3. In the window that opens, tap the Edit button
4. Edit the description, logo, type, and level of the guild
5. Tap the Save button to activate the changes. If you change your mind about changing the guild parameters, close this window.