Start outside the wall
Window leaks are often tied to flashing, house wrap laps, trim joints, or siding details around the opening. The repair should address the path water is taking, not only the interior stain.
Ceiling stains can be misleading
Water can travel along framing, insulation, and drywall before showing up indoors. A ceiling stain near a window may still originate above the window, at a roof-wall intersection, or behind siding.
Repair the opening as a system
Depending on the cause, the fix may include window trim removal, flashing correction, siding repair, sealant replacement, or window replacement if the unit itself has failed.
A durable repair starts by identifying the leak path before closing the wall again.