How to straighten horizon mirror on sextant

Hello,
The horizon mirror of a Tamaya sextant which I recently borrowed is not straight: The boundary line which separates the coated from the non-coated half is not parallel to the sextant frame, see the area circled in red in the pictures attached of the front and rear of the horizon mirror.

How am I supposed to straighten up the mirror?

Thank you! :grinning: