You need to have two app: xev and xmodmap, both are commandline-tool.
Run xev. Then press the key whose behaviour you want to change. i.e. Enter. and then
xev will output some information about the pressed key. Pay attention to the third line. It should be like this:
state 0x10, keycode 110 (keysym 0xff55, Prior), same_screen YES,
In this example Prior is the name of the behaviour the key is assigned to at the moment, the number keycode is the internal id to recognize the key. Now do this with another key i.e. PgDown give this output
state 0x10, keycode 115 (keysym 0xff56, Next), same_screen YES,
Here again the interesting part for us is keycode 115 and Next. Now, when you want to swap the two keys use xmodmap.
xmodmap -e "keycode 110 = Next"
This changes the key with keycode 110 on your keyboard to the action Next. It's pretty simple.