The Upgrade Benches are scattered across the Battle Royale map. They were added on the initial patch of Chapter 2 Season 1. It allows you to upgrade your weapons in exchange for materials, like the Vending Machine. One can be found at all named locations and a large number of landmarks around the map.
In Patch 13.20, you can now obtain Upgrade Benches from Chests, Supply Drops and Supply Llamas. You can throw them and reuse them by breaking them, they have 400 health. Normal Upgrade Benches that spawn can now also be destroyed and dropped, they also have 400 health. It appears as the Rare rarity.
The itemized Upgrade Bench has been vaulted on Patch 14.00 as you can no longer break them. However, the itemized variant remains avaiable in Close Encounters LTM.
The Upgrade Station is no longer available as of Patch 15.00 and is most likely replaced with the ability to upgrade your weapons with the NPC's on the map.
Instead of upgrading a gun, you can sidegrade it. What it does, is that you can for example take an Assault Rifle and sidegrade it into a Heavy Assault Rifle without changing any rarity. Throughout seasons new sidegrades are possible. Each side grades costs 20 Wood, Stone & Metal. Though, as of Chapter 2 Season 3, you can no longer sidegrade an AR due to the Heavy AR being vaulted at the start of the season.
Sidegradable weapons change almost every season because of the weapon vaults and unvaults.