How to enlarge grommets
Grommets are typically thin wall tubing with the ends peened over to form the flange you see in the photo. Any tool used to grind away at the inside diameter will remove most or all of the thin wall tubing composing the grommet.

The primary option you have is to remove the existing grommets and purchase a grommet installation kit for the size you need. It's an anvil smaller than a hockey puck and a driving tool. The grommet half is placed through the hole, then set on the anvil with the other grommet half on top of the fabric. The driving tool has a recess and a point to match the inside and outside curves of the grommet. A solid strike with the hammer provides the bending and bonding force.



There are other types of grommet installation tools resembling mechanical presses with much higher prices. For the occasional or hobbyist use, the hammer method is more cost effective, with the slight penalty of inconsistent results.