By default when you add a new AP to your UniFi Controller it will be adopted using the default site. Then, if you want to move it to another site, you can do that following these steps:
1) Go to the Access Point tab
2) Find the Access Point you want to move to another site, and open the Properties tab (click on the device’s MAC address or the name for the UAP to open that tab)
3) Select Configuration and go to the “Forget this AP / Move” section
4) Select the new site where you want to place this specific UAP
Select the access point and click “Forget this UAP / Move” to move it to another site.