The trust levels look like 0->1->2<->3, 4. You can’t go back down from 2 (without an admin explicitly changing it) but you can go down from 3 to 2. 4 is only available manually.
In order to reach and maintain trust level 3, a user must view/read/reply to topics and give/receive likes to whatever amount over a period of time, as well as visit some number of days within that period. All of those values can be set by you in the Trust Level settings.