On the left hand side there is an editor tab, select that and at the bottom you will see world tier.
Yep, tiers have to be unlocked one by one. Maybe it's possible to manipulate the points to next tier to unlock them faster or the challenge that are needed for that unlock points..
[Edited by madatcheats, 6/30/2022 9:32:20 AM]
Correct, all the 3 options under World Rank Progression display ??, and the World Tier change itself is only visual, doesn't change the enemy ranks or the loot drop.