Without a LOT more detail no-one’s going to help you.
Even though I completed some BHI training courses in 2010 I learned nothing about changing crystals, I’m totally self- taught.
A lot of it us down to common sense, if you’re dealing with mineral or sapphire crystals that fit in a gasket it’s essential to get the correct size of replacement, these glasses are sold in 0.1mm increments and you must get the correct size, a good quality digital calliper will help with the measuring. If you can’t be certain of the size buy 2 or 3 in different sizes and choose the one that fits.
Acrylic crystals are totally different, tension ring crystals are supplied in 0.1mm incremental sizes for around £5 each, the correct size will press in using a crystal press without excess force, but sometimes you have to buy 3 to get the right one.
I’ve spent more than enough time replying, if you want more advice you must give more info, lack of info is laziness on your part! Don't be fooled by u- tube videos, they always make things look far easier than they really are.