This is a fairly common question from users who return to the site after taking a medium / long break.

Keep in mind you can let the SRS do its thing - and that you could also configure the Hard rating to drop flashcards to the 1st box - no matter what their last review interval is (you can do so in SRS Settings > Maximum box for cards marked 'Hard'). The advantage of this method, as opposed to using No rating is you don't have to go through the Restudy cycle. And, as long as you keep rating Hard on cards that are in the 1st pile, they get a fixed 1 day interval.

This would be the recommended way, allowing you to keep the longer intervals on the simple kanji that you don't need to study again - while "resetting" all the other ones to the lowest interval.

If you really would like to "start over" : see Resetting Flashcards in the F.A.Q.