Rename your Drawing by tapping the name below the drawing in the Gallery.

 

Rename a Folder by tapping its name at the top of the Gallery.

 

As the Gallery Sidebar uses the tap gesture to move between folders, use tap+hold to rename a Folder in this area. 

 

As Subfolders inside a Folder use the tap gesture to hide or discover drawings, tap+hold a Subfolder name to rename or delete it.