have MW2 open, Also have Cheat Engine Open (doesnt matter if steam or cracked version)

open IW4MP.DAT by clicking the top left button in CE and selecting IW4MP.DAT

Get you XP Value (lets say mine is "999") Memorize it/Write it down
Go on cheat engine and type in your value (i would type in "999")
Go and get abit of xp to change the value in the game; then type in your new value and click NEXT SEARCH.
You should only get a couple of GREEN VALUES(double click them to get them into your area box at the bottem and then change the VALUES from there) change these untill your xp changes (note every time you change a value look to see if you xp has changed)
Once you have found it change it high enough to get you level 70.

now depending what prestige you already are type into cheat engine that (so say i was 5th Prestige and had just modded my xp to be able to Prestige) value so lets say im 3rd Prestige so i type in three, i then click FIRST SCAN! it brings up aload of results

I then go into the game and prestige... i am now 4th prestige!

now go into Cheat Engine and type 4. Click NEXT SCAN your should get a couple of GREEN Addresses change the Values on them (Checking Inbetween) And change it to whatever prestige you want.
You can then set your self level 70 as you still should have th XP Addr!