declare
  @CSL_Priority int = 2,
  @CS_ID        int = 3521


update csl 
set 
  csl.CSL_Priority = @CSL_Priority

from CS_ControlSystemList csl 
inner join CS_ControlSystemList s on s.CSL_ID = csl.CSL_ID 
  and s.CSL_ControlSystemID   = @CS_ID 
  and s.CSL_PersonID          = 21067