Manage site collection administrators
This article describes how global admins and SharePoint admins in Office 365 can add and remove site collection admins for any site collection. If you're an owner of a communication site, or a site that belongs to an Office 365 group, see Manage your SharePoint site settings for info about giving people access to your site. If you're a site collection admin for a classic site, see Manage your SharePoint site settings.
If you're a global admin and want info about assigning other users the SharePoint admin role in Office 365, see Assigning admin permissions.
Add or remove site collection admins
If a site belongs to an Office 365 group, you can add or remove group members in the Office 365 admin center. For info, see Add or remove members from Office 365 groups using the Office 365 admin center.
If you want to change the admins for a communication site or a site that belongs to an Office 365 group, and you as the global or SharePoint admin are also an owner of the site, see Manage your SharePoint site settings. If you aren't a site owner, you need to use PowerShell to add or remove site owners. For info, see Add-PnPSiteCollectionAdmin and Remove-PnPSiteCollectionAdmin.
To add or remove site collection admins on classic sites, use the SharePoint admin center:
Sign in to Office 365 as a global admin or SharePoint admin.
Select the app launcher icon in the upper-left and choose Admin to open the Office 365 admin center. (If you don't see the Admin tile, you don't have Office 365 administrator permissions in your organization.)
In the left pane, choose Admin centers > SharePoint.
Point to the site collection for which you want to change the administrators, and then select the check box in front of it.
Click Owners, and then click Manage Administrators.
Change the name in the Primary Site Collection Administrator box, or add or remove names in the Site Collection Administrators box.
Click Check Names to verify that the user names are valid.
Click OK .