ok its easy.... just download a lvl hack here on the forum (use search function)
Than delete all ur .stat files in the mw2/players directory and open the hack/trainer
Now use the Trainer how in the discription and voilà U r lvl 70
People have already said to delete the .STAT files in the players folder. After that, create a class or something, and the screen will freeze for a second. Don't worry, its only saving your new stats. Works with Weapon attachments too.