Ok guide:
First thing you will need is a couple of screwdrivers, they generally look something like this:
Now get an overview of your pickguard, do you see any screws? Screws are what hold the pickguard tight against the body. These need to be "unscrewed" to remove the pickguard from the body.
The screws are most likely phillips type screws, so make shure your screwdriver is of the same type. The screwdriver also needs to be the right size, and since there are probably at least two types of screws on your pickguard (the pickups are also fastened with screws, but of a different type). Screws are the little metal things with slots on the heads, these correspond to the tip of the screwdriver wich will enable you to unscrew them. Here is a picture of a screw:
Step 0: Remove the strings
This step is not necessary, but will save you and the guitar a lot of pain. If you do not know how to take the strings off, google it or have a proffesional do it for you.
Step 1: Loosening of the pickguard
To loosen a screw, hold the tip of the screwdriver firmly pressed against the slots on the top of the screw's head. Then, while pressing firmly, rotate the screwdriver in a counter-clockwise motion. When the screw is unscrewed it can safely be removed from the guitar by hand. Put the screw away in a safe place where it won't be lost. Now repeat this step for all the screws holding the pickguard down around it's perimeter.
When all screws have been safely removed, you can feel that the pickguard has come loose.
CAUTION Do not remove pickguard before completing step 2 CAUTION
Step 2: Loosening of the pickups
If you do not want to damage your electronics you have to remove the pickups from the pickguard before fully removing the pickguard. Here's a picture of a pick-up:
The pickup is also held down by a set of screws and some springs. Here is a picture of some springs:
When the pickup is fully unscrewed the springs will come loose, so you have to keep an eye on these buggars because you don't want to lose them.
Step 3: Removing the pickups from the pickguard
When the screws are fully unscrewed the pickups can be removed from the pickguard.
Step 4: Removing the pickguard from the body
You are now probably holding the pickguard in your hand, put it away.
Step 5: Putting the new pickguard on the body
Take the new pickguard and place it on the front of the guitars body, making shure the screw holes match up, if they don't bring out the drill or slap yourself and buy a pickguard that fits.
When you've checked that it matches the body or put a couple of extra holes in it, it's time to put the pickups on the new pickguard. Do this just like taking them off the old one, except this time turn the screwdriver clockwise. After the pickups are fitted it's time to screw the pickguard to the body of the guitar. This is done exactly like unscrewing them, except this time also in a clockwise motion.
Step 6: Put the strings back on and play