First off you need a fully upgraded ib weapon. Next you need to dismantle it, which will give a coin of iron. Go to Lord Saladin with this item in your inventory and you should be able to exchange it(along with 2 motes) for a new version of your gun, only this time with different perks. Note: this will also bring 300attk max weapons to 331. It also has about a 10% chance to grant your weapon a new element(even on primaries like timurs lash). You can even get the perk firefly on hand cannons when you reroll.

Go to Lord Saladin with that weapon equipped and an option to reroll should pop up. It can only be done with Iron Banner weapons.

this only really applies to Iron Banner legendaries. If you give your IB weapon + motes of light to Lord Saladface, he'll give you back the same weapon with different perks.