First before you change it's status, make sure you have enough licences to make this a managed devices again:
from manage > clear all filters, then in the "device management state" filter section, expand it by clicking more and then tick unmanaged. At the top you'll see how many devices this is. Then check on licence count by clicking on the name in the top right > global settings > licence management.
If you've got spare licences jump back to the manage page, clear the filter, find the AP, click the tickbox on the left hand side > actions > change management status > manage
