Mess Ikonta 524/16 rangefinder vertical alignment

I've got a Mess Ikonta 6x6 folder which I've had a couple of years, bought it with serviced off eb-y. It's worked well, but the vertical alignment of the rangefinder has slipped to quite an extent. Does anyone know how simple / difficult it is to fix this?

Thanks in advance.

Should have mentioned, it's an uncoupled rangefinder.