Before you attempt to replace the battery you should first check the troubleshooting guide to ensure you need a battery replacement.
To remove the battery cover, push in and downwards towards yourself on the cover. There is a notch on the back that you can use to get a better grip if necessary.
Pull the battery away from the battery housing. At the end of the wires on the housing side there is a clip that connects the battery to the phone. Unplug this clip and discard the warn battery appropriately.
Now take your new battery pack and insert the new plug into the housing. The plug should smoothly clip right into the phone.
If the clip does not fit easily into the housing, compare the new battery with the old and be sure they are the same.
Push the wires back into the housing to allow the battery to fit under the battery cover.
Put the cover back on. Listen for a click, which will indicate the cover is placed correctly.