To trade in items for a new item go to your Garage, select manage inventory. From here you will be able to choose the items that you would like to trade in.
Please note there are a few limitations.
- DLC purchases cannot be traded in.
- Crate items must be traded in with items from the same series of crates.
- The highest level quality items are not tradable.
- If you trade in an item that was unlocked through a Decryptor, the item received will also be trade-locked. You can read more about Decryptors here.