Mods can be used on units that are level 50 and above. Equipping a mod on a unit, and upgrading a mod both cost Credits.
You can earn mods by spending Mod energy in Mod Challenges and Mod Battles These also open up to you at player level 50). You can also get Mods as rewards in Raids and other group content or events, or you can buy them from the Mod Store in Shipments.
There are different types and star levels of Mods. Tap on a Mod to see its base stats, one or more of these will increase when you upgrade the mod. Slicing a Mod that isn’t already Gold will randomly upgrade its stats. You can find Slicing materials in Tier 9 Mod Battles, or buy them from Shipments.
Mods work best in sets, with a specific number of them giving your unit an additional set bonus. Using a particular Mod in a particular slot can make an extra difference too.
Manage your Mods through the Mods tab in your Character inventory. Tap on a unit here to assign or swap mods. You can also create loadouts (tap the Loadouts button) that you can lock to a particular unit or move from unit to unit for a fee in Credits.