Most of the time, use capital letters instead of uppercase. It's OK to use uppercase when comparing with lowercase or to follow the UI.
One word. Don't use uppercase as a verb. Don't use uppercased.
When using uppercase and lowercase together, don't use a suspended hyphen (as in upper- and lowercase). Spell out both words instead.
Examples
Change all the lowercase letters to uppercase.
Change the case of both lowercase and uppercase letters.