How do I change until the next underscore in VIm?

You can do cf_. f won't highlight the searched character.

You can also do ct_ if you don't want to include the _.

Put this in your .vimrc:

set iskeyword-=_

Then _ will be treated as a word boundary (though not a WORD boundary), and cw could be used to just change "awesome", and cW to change the whole thing.

See:

:help iskeyword

and

:help word

for more info.