I take no credit for this as I did not find it. All credit goes to the founder Gears of Hell.
I confirm this works as I just did it.
I would personally suggest that you mod to level 99 & change your kills/deaths etc to twice as much as they were when you were first level 100. That way it seems more legit if they do check. My Current stats at level 100 reup 1. (Green level 100);
Matches won 2607
Matches lost 1941
Matches MVPs 610
Rounds drawn 270
Rounds Lost 9898
Special Events Played 11
Total Score 2065343
I have also gone the extra mile of making sure the amount of games including wins/losses/draws for each game mode equals up to the amount of games I have lost/won overall. Remember this is modding & there is always a chance of getting banned.
[CENTER]1. FIRST THING you need a USB flash Drive
3.Now Back -up Profile and Player Dat DATA
4.have you profile and Stats on your USB FLASH DRIVE
5. Now Start Up GEARS 3
6. load all your data from your USB ,Stay on the Menu Screen
7.now it should show your Main rank witch (if you RE-UP ) its low like 20 rank
8.Unplug your USB go on your Pc And mod your Stats on Horizon Save it and Plug it back in Your 360
9.Now you should see your Screen saying Continue without saving or find a new place to save
10. Pick find place to save . Now Load up Your USB ..and select USB MU
11.Now it should still say your whatever rank you had before that fine now Sign out Sign in
12.Now it should be the level you modded it too.