(I get a lot of e-mails asking this question.)
Click Start -> Control Panel -> Add or Remove Programs -> Check the Show update box at the top of the dialog -> Scroll down and click Internet Explorer 7 Beta 2 Preview -> Click Remove.
The answers to this question and more can also be found here.
And, while we're on the topic of the preview, please remember this is a technical preview. It should not be installed on mission critical machines in production environments. Please download it, play with it, and tell us what you think, but please use a test machine and always back-up your important data.