You need to change three things for the particular unit you want to change.
1. The most essential step is to change the animation level of your unit to something above 5, which will allow them to walk over all terrain. If you're unsure what to put here, set it to 16. You can assume all ground units start with a animation level of 4, so adding 12 will work for this.
2. Change the unit's movement type to include air and hovering. That means, for a ground unit you can add 4 to their movement flags to add air movement. For a unit that does not hover (workers, vulture, archon, high templar, probably more I forgot) add 128 to their movement flag to make them into a hovering unit.
3. You need to update their special ability flags to make them an air unit. This step is optional if you still want the unit to be targetted like a ground unit. Assuming you've already got a ground unit, adding 4 to the special ability variable will do the trick here.
I would tell you how to change attack range if it weren't blindingly obvious. You need to learn that we can't give you step-by-step instructions for everything, especially for changes that stare you in the face. Take a look again at MemCalc under weapons.dat and you'll know exactly what to change.