Are you looking for how to put in like a PCI or ISA or AGP card? I'm making a large generalization here, but motherboards have 3 "upgradeable" points: the CPU, the memory, and the I/O PC slots. I assume you're talking about PC slots. These also come in different type: ISA, PCI, PCI Express, and AGP. Newer motherboards mostly come with just the PCI and PCI Express slots. Replacing boards is fairly simple:
1) Make sure the board you're replacing matches the slot type
2) Ground yourself (which can easily be done by touching the metal frame of the chassis
3) unscrew "holder" on the old board and take it out (carefully)
4) insert the new board and screw in the "holder"
5) install software for the new board if needed
Opening the case varies from case to case, some have a sliding door, some have a swivel door, some require on or two mechanical buttons that need to be pushed before, or while opening the door, you just have to figure that one out yourself.
here's some pictures of slot types:
http://www.worldstart.com/tips/tips.php/992http://en.wikipedia.org/wiki/PCI_Express
