As your character increases in power, they can choose to learn new abilities (i.e. taking Feats to gain new spells or abilities), or they can choose to enhance the power of the abilities they already have. This second path allows you to focus on a core set of abilities, the ones that you use regularly in combat, and make them better, more powerful and more versatile.
That second path is what Upgrade Feats are for.
A Feat with the Upgrade tag is applied to a single other Feat or Aethertech Device, specified by the Upgrade Feat in question.
Upgrade Feats all require at least Tier 2. Some of them require Tier 3, and a very rare few require Tier 4.
You can only apply an Upgrade Feat to a given feat once, unless the Upgrade has the Ranked tag, in which case it will list how many ranks you can take of it and how you can apply those Upgrades.
Upgrades are intended to represent significant time and training with a given ability or Device, so you can only apply a single Upgrade Feat to a given feat or Aethertech Device over the course of each Tier. This does not function retroactively; if you don’t apply an Upgrade to a feat in Tier 2 and 3, you can’t apply three of them all at once in Tier 4. (However, if you are rebuilding your character entirely, you can ask your GM for permission to retcon your character’s history, rewriting their story so that they’ve been training with the ability or Device for more of their career.)
If an Upgrade has the Ranked tag, you can take a number of ranks of it each Tier equal to the number of ranks of the feat it is Upgrading. Like the non-Ranked version of Upgrades, this is not retroactive once you go up in Tier.
Combat Feat Upgrades
Elemental Affinity Upgrades
Skill Feat Upgrades
Upgrade Feat Pages