Each tower has four upgrades, one from each category (turrets, barrels, area effects, and defense upgrades). The tower repair falls under the defense upgrades category. If you are lucky, at least one of your towers will start with it. If you want, you can spend money to reroll the defense upgrades at each tower until it becomes tower repair (you do this by holding interact on the upgrade, it costs quite a bit). The other upgrades in the "defense upgrades" category are "airstrike", "knock back pulse" and "heal pulse". So if you see one of those you can keep rerolling it until you get "tower repair"
How do you heal the towers?
Nvm, I finally got my victory pose :)
Each tower has four upgrades, one from each category (turrets, barrels, area effects, and defense upgrades). The tower repair falls under the defense upgrades category. If you are lucky, at least one of your towers will start with it. If you want, you can spend money to reroll the defense upgrades at each tower until it becomes tower repair (you do this by holding interact on the upgrade, it costs quite a bit). The other upgrades in the "defense upgrades" category are "airstrike", "knock back pulse" and "heal pulse". So if you see one of those you can keep rerolling it until you get "tower repair"