Summary
Results for Polls, Quizzes, and Surveys are calculated when needed using form entry data. This means to reset your results, you cannot just reset a counter, but you must delete the entry data associated with the fields of your Results type.
There are two primary ways to delete entry data: delete the whole entry, or by delete the field itself (along with all he associated data).
Reset by Deleting Entries
You can trash entries via the Entry detail page, or you can trash either individual entries, or entries in bulk, via the Entry list page. See the short animation at bottom.
Consider exporting your entries before deletion if you would like to keep a copy the existing data, just in case!
Reset by Deleting the Field
If you have a form that has field types that do not support results, (that you would like to keep), then deletions become more complicated. Instead of deleting the entries, we recommend editing the form to delete the fields of your Result type, i.e. Poll, Quiz, or Survey, which would then delete the entry data for those specific fields. The data from any other fields would remain intact. You can then rebuild the form by adding back the Poll fields as new.