Sorry noob question. How do you upgrade ammo?

Do you mean how do you equip special ammo (e.g. disruptor or cryo ammo) or how you increase ammo capacity?

Special ammo requires an ammo power in single player (just press the power button to apply it to your current weapon) or use of a one-time consummable in multiplayer. In single player you can also have allies upgrade their ammo powers so you benefit too, but that's not as potent as using your own ammo power. Most single player classes have some kind of ammo power but you can also choose one as your bonus power.

Ammo capacity can be increased by upgrading the weapon in question or equipping capacity-boosting equipment. There's the odd passive ability in single-player and multiplayer, but those are the main ones.
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DF: So why can a Marauder roll but a multiplayer Turian can't?
Delta1-9: You are fighting female turian husks. You have reach but they have flexibility.