I have one with a Kalart Rangefinder, but it's mirror seems to be unusable. I got a new mirror, but the old one is glued very well, so I can't remove it.
What can I do to remove the glue?

Denatured Alcohol applied sparingly around the perimeter of the mirror frame may work. You may have to let it soak in for a day or two. It is commonly sold in the paint department of home improvement and hardware stores.

Check your local stores for an adhesive remover. Goo Gone leaves an oily residue that is hard to clean off.

A hand held hair dryer on high setting may soften the glue enough to get the mirror off.

Thank you for your advices.
I have melted the glue by the soldering iron (480 degrees of Celsius).