how to replace the ram?

You can add or upgrade Ram. You can do it yourself, but that is not advisable because of the difficulty involved in gaining accessing the Ram slots.

You can find instructions on the Internet that will show you how to take the Mini apart.

You don't mention which model you have but here is a short video on the Intel Mini...

http://eshop.macsales.com/installvideos/macmini_06-08_mem_hd_optl/

I have taken mine apart several times and haven't had any problems adding memory.

As posted, you can do this, I have upgraded both the memory, and replaced the hard drive. The video referenced will give you a good idea. The secret is to have a brand new putty knife with a thin blade. You may want to grind one edge of it so there's a thin edge on it. After that, you'll need a small phillips head screwdriver. Also, you need to work slowly and carefully. Note what you're doing, and watch unhooking any wires by mistake.