There are several methods to rename a file in Final Draft Cloud.
Renaming from Final Draft Vault:
In the Vault interface, locate the file you would like to rename, then click the three dots on the right of the file. Click Rename, and then type in the new name into the window that opens:
Renaming from Final Draft Writer
With your file open in Final Draft Writer, click the three line menu in the top left, select File, then select Rename. You can then type in the new name into the window that opens: