Just double click in the column that says "Value" and enter a number, then hit enter. Most changes take place instantly. Some changes require that you play a match after changing the values in order for it to stay.
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cluckt Sorry that I couldn't get to your question sooner. I see that Jorndel seems to have solved your problem.
For future reference, messing around with the weapon xp or prestige values is not recommended unless you are fairly good with Cheat Engine and know what you are doing. I only put those weapon xp addresses in this file for completeness. As you can see, I only put the weapon prestige address in there for one or two weapons. I shouldn't have even put those in there.