fastest way to level in diablo 3 season 12

Are you trying to upgrade it by getting a set, just stat upgrades or a specific item? All of these would be valid for 'upgrading gear'. Gear upgrades could have multiple meanings.

Upgrade gear is not a very good term to use in Diablo. After this, its the long grind of slowly getting better mods on your gear and pushing your way up to T13 and high GRs, upgrading gems, etc.

Once you are using your class's best set and have the right legendary gear in every slot, you should be easily T10 capable at this point.I use blood shards to gamble for armor pieces, and kanai's cube to upgrade weapons for the one I need. Start farming for the best set of my class.Even level 1 gems can be a huge boost in damage if you pick the right ones (bane of trapped, zei's vengeance, bane of powerful, etc). Also, completing the focus and restraint ring set is usually a huge damage boost. I typically use the kanai's cube to upgrade rare weapons into legendaries for the specific one I need.

Complete the other items required for the class set (jewelry, weapons, etc).

If you are on seasons, completing season journey chapter 2-4 will give you a free set. Pick the set you are closest to completing after doing step #1. Any complete set, even the worst set, is drastically better than having no sets at all. Once you have a complete set of legendary gear, no matter how bad, you can handle torment 1 no problem. To do this, I will gamble using blood shards on every slot that doesn't have a legendary in it (other than weapon, I craft those). Here is my typical priority while starting a new season (similar to starting out completely new):

Fastest way to level in diablo 3 season 12