Instructions on how to change a tree name in Family Tree Builder can be found here.
Note:
If the tree is currently opened, you cannot change its name.
If there is already a tree with the exact same name, you won’t be able to use the exact same name for another tree.
Instructions on how to change a tree name on the family site online can be found here.
Note:
The tree should have been synced at least once (more info about sync can be found here).
If the tree was originally created within Family Tree Builder (FTB), you have to change the tree name in FTB and then sync to the family site.