In MS Word, how do I un-indent a bullet?

Pressing Shift+Tab will do this.

In addition to the other answers, there are toolbar/ribbon buttons for "Decrease Indent" and "Increase Indent".

Oh, and one more - I believe if you hit Enter when on a blank item in your list, it decreases its indentation level as well.

Use Shift + Tab to step back out of the indentation.