To remove the mirror in the Yashicas, simply slide it up. There is a metal clip on the back/top, thin metal about a cm wide. I use something like a toothpick to hold the clip down. Then push up from the bottom- I use a wooden chopstick from take-out. Try to avoid touching the front of the mirror, in case you want to reinstall it.
Cleaning the front-surface mirrors on TLRs is something that I haven't had much luck with. I'd search here for others' suggestions.
To reinstall- DO NOT touch the front! I wear a plastic glove on the hand that will handle the mirror, and hold only by edges, upper sides (no powder on glove). I use a cotton swab or a chopstick with a kimwite taped on the end for pushing the bottom edge into the groove.
The grooves that establish the mirror location are cast into the body- no adjustment even possible. With the spring on the back and the front surface mirror riding against the upper surfaces of the grooves, focus should not shift at all.