C103 and others
The heated toilet seat is fixed to the toilet bowl with the white bolt on the back of the red circle.

Turn and remove the white nut in the red circle. Also remove the white washer ring and black gasket. (2 places) Once removed, the heated toilet seat will be able to move.

Lift the toilet seat and turn it over. There is a grey drain screw on the right side.

When you unscrew the drain screw using a 1 yen coin or similar, about half a cup to a full cup of water will come out of the hole, which you can then pour into the toilet. (Be careful not to drop the screw you removed.)

Once the water has drained, return the heated toilet seat to the toilet. Pass the bolts from the heated toilet seat through the two holes in the toilet.

(View from under the rear of the toilet seat) Pass the removed black gasket and white washer onto the bolt from below, then turn the white nut to tighten it. (There are two places.)