You will need to be an owner or admin to do this.

Step 1: Click on your Dashboard (gold box and #1 in image below).

Step 2: Locate the Sitemap section (fuchsia box and #2 in image below).

Step 3: Click on Users (turquoise box and #3 in image below).

Your users page will open.

Step 4: Hover at the right-hand end of of the user whose number you wish to edit. If an Edit appears (turquoise arrow in image below), click on it. If a 3-dot Action Menu appears instead (depend on the user role), expand and click on Edit User.

Step 6: Choose the number you wish from the dropdown menu. Click Save to complete your action.