Go to Settings (the Settings icon), Applications, Manage Applications, select your application, click Uninstall, then answer the confirmation questions.
Maybe instead of "Uninstall", the button says something like "Unstall and Refund". I don't currently have a paid app to test it with, so I'm not 100% certain of the details.
But that's the general idea.
I hope it's not a "Widget", because I don't fully understand those.