You can switch from Tribal to Feudal in CK3 by entering the decisions menu and meeting the following requirements at any time: You have the Law Absolute Tribal Authority. Your religion is an Organized Faith. Your level of fame is Distinguished (Level 2) or above.
A huge plus of Norse paganism is being able to prepare invasions and pouring prestige into tribal armies. I would say that the advantages of both government types are contextual, but as time goes by, feudalism becomes better and better, as tribalism is limited to inferior light infantry.
Click on the “village” holding (marked with a green arrow and USUALLY the top holding) and you’ll get a popup like the one to the right below. The Earth Hillfort in this holding is already Level I so will “only” take three more upgrades to become a Stone Hillfort, which is what you need to go feudal.
Assuming you start as a tribe, you’ll need to increase your tribal authority all the way to level four. At that point you’ll have the option to adopt feudal ways or clan ways. This will let you move up to the crown authority you need to change the succession laws.
Adopting feudalism Converting tribal settlements later (by right-clicking) requires that the province shares your religion. Your tribal vassals will be able to join you in feudalism as long as they follow an organized religion. Vassals are now required to construct a Stone Hillfort to take the decision.
Regional innovations are unlocked when the Culture has a sufficient presence in a certain Region, and remain unlocked even if said territory is lost. Cultural innovations are always active, but require a certain Culture.
As a point of interest, you can lower the crown authority afterwards without losing the succession law. Crown authority can be increased by a ruler so long as his realm vote it through, this can be done multiple times supposing a ruler deem it worth the long wait in between.
How to gain prestige in Crusader Kings 3 Starting wars and winning. Assisting in wars with allies and winning. Performing decisions, such as having a Great Hunt or a Feast. Having a higher diplomacy skill. Raiding an enemy territory. Doing something heroic.