You can promote an existing admin to become the Primary Admin (page owner) of the page.
To do this, visit your page and click the "Settings". Once there, go to the "Page Admins" section.
Click the "Edit" dropdown next to the name of the admin and then click the "Make Primary Owner" option.
When you click this, you will be prompted to confirm this action.
Please Note, this option will promote this user to be a primary admin and they will have the option to remove you if they want to.
Once you confirm this action, they will become the new Primary Admin.