Upgrade Tokens are a secondary economy system required to upgrade your island level. why Upgrade Tokens? Tokens are more difficult to obtain which prevents players from easily upgrading islands on alt accounts and requires you to grind further to unlock Island Levels. The required amount of tokens for level upgrades will be shown in Island Level GUI below the cash cost.
The main source of Upgrade Tokens is from completing Expeditions in the Expedition World. You will complete various tasks and be rewarded with various items as well as every few levels you will be rewarded with Upgrade Tokens. Tokens can also be obtained from crates, Most common in event crates which can be obtained from completing events on Nebula: Spleef, Sumo, Dropper, Etc.