Can not remove the Devel module
Can't you use the "normal" upgrade scenario for FTP servers?
(I usume this still works btw. haven't tested it on d8 yet)
 - Download your intended version of devel from d.o.
 - Unpack the download
 - Upload to the server in place of the old devel module (so same paths,permissions,etc)
 - Run the "update.php" script to update the Database.(you need to be admin to do this, or change your settings.php to temp. allow anyone to do it).
 - Devel should be updated now.

according to the docs this should work see d.o. D8 Update a module