Sometimes you may want to "reset" part of your collection so that the cards become new again and the scheduling algorithm starts from the beginning. For instance, you might have completely changed a card or updated it so that you need to start learning it again, or you may have flipped through some cards without really studying if you were just trying to take a look at the cards or figure out how Anki works.
Resetting part of your collection because you are behind is not recommended. While it can be tough to recover from a backlog of cards, erasing all the progress you have made is the worst possible thing you can do: you will still remember many of those cards, and you'll have to start them all over even though you could have pushed "easy." Instead, consider setting up a daily review limit ("maximum reviews/day" in the options) and studying a set number of the overdue cards each day until you catch up.
- Find the cards you want to reset in the browser.
- Select all the cards and choose Cards->Reschedule or Edit->Reschedule, then Place at end of new card queue. Click OK.
When you do this, Anki will turn the cards back into new cards. If you click Info in the browser you'll see any previous reviews you've done listed there, but that history will not influence how the cards are scheduled: they will be treated just like new cards.
If you accidentally imported the review history of someone else, because they shared their deck with scheduling, you can use the following add-on to remove the history of cards you have selected.