To upgrade Performance parts, you need to collect multiple copies of the same part and spend a certain amount of Tokens. When you receive a copy of a part you already own, it is automatically stacked in the R&D inventory.
All parts' maximum level is level 10.
The amount of Tokens required to upgrade a part varies according to the rarity and the upgrade level of the part. The rarer your part is, the more Tokens you will have to spend to upgrade it to the next level. The higher the upgrade level of the part is, the more Tokens will be required.