From your News Feed, click Pages in the left menu.
Go to your desired Page, then click Settings in the left column.
Click Page Roles in the left column.
Type a name or email in the box and select the person from the list that appears.
Note: It can be hard to find the correct person with a given name on Facebook. Add someone as a friend first to make them appear at the top of the dropdown results.
Ensure that Editor is selected from the adjacent dropdown menu.
Settings in the left column.
is selected from the adjacent dropdown menu.