Siezed bleed nipple prevents conventioanl bleeding , so :-

Open caliper to expose piston
Crack open caliper supply line
Push piston back into bore thus evacuating caliper of air/bad fluid
Open reservoir
Allow to gravity bleed
Close caliper supply line
Close and bolt up caliper
Top up fluid as required
Slowly press brake pedal to reseat piston on pads

What do you reckon ?  (TIA)