Before changing case, you'll need to select the text you're interest in.
To do this, start on the first character you want to toggle, then enter visual mode by typing v. Use the directional keys (h, j, k, l) to select the rest of the text you want to change.
To toggle the case type g~.
If you want to make the text all uppercase use gU.
If you want to make the text all lowercase use gu.