How can I change the Cost Center for the GL Account?

Once a GL Account is created, the cost center can't be updated.  You will need to create  a new GL Account with the correct Cost Center.  For the incorrect GL Account, that should be deleted if if there are no transactions attached to it or it should be marked as posting prohibited if there are transactions associated with it.  Marking it posting prohibited will remove it from the Chart of Accounts query and disable the ability to post to it.