First you need to understand what put you overweight.
You can go overweight from two different reserves, Health Reserves or Stamina Reserves.
So you have to get your reserves to drop, only way to do that is get them down to zero, then when you have grey bar, rest that off or eat something to get it gone but not positive. Then take a lot of damage, like jumping off the bridge in MK, or the Cantari Keep Brige, or the top of a Keep, or a tier 3 Stronghold, or drowning.
If you went over due to stamina reserves you can just use a over strength longbow and keep drawing it, and not firing it. Just holding it till your stamina is gone.
The last way is through weight loss potions. They generally drop you 1kg per 10 units.