Search Center settings
As a global or SharePoint admin in Office 365, you can specify where searches should go for your classic site collection or site by specifying the URL of your Search Center. For example, if you have created an Enterprise Search Center where users can search everything in your company, you can enter the URL of that site here.
Enter the address of your Search Center
Sign in to https://admin.microsoft.com as a global or SharePoint admin. (If you see a message that you don't have permission to access the page, you don't have Office 365 administrator permissions in your organization.)
In the left pane, under Admin centers, select SharePoint. (You might need to select Show all to see the list of admin centers.)
If the classic SharePoint admin center appears, select Open it now at the top of the page to open the new SharePoint admin center.
Select Classic features.
Under Search, select Open.
On the search administration page, choose Search Center Settings.
In the Search Center URL box, type the URL of the Search Center site.
It may take up to 30 minutes before changes take effect.