This procedure worked for me, on Piwik version 2.9.1 . It should work on more recent versions too.
Use your favourite MySQL browsing and administration tool, such as phpmyadmin (I use mysqldumper) to change the “users” database table. Find the row corresponding to the user to whom you want to give Superuser rights, and then change the “superuser_access” field from zero to one.
If you are uncomfortable with manually changing the database contents, you should ask your service provider to do this for you. It’s a trivial change that’s very easy to recover from.
Once you’ve changed the database, you should be able to log into Piwik with that new Superuser ID, and change the password of the old Superuser or delete that old Superuser completely.
I hope this helps.