This isn't really advised - generally it's better to have the mindset of "the algorithm knows better" - the algorithm behind Mnemosyne is well researched and evidence based.
You can reset the dates for cards, not through Mnemosyne, but by modifying the card database using SQLite - though this is incredibly dangerous and may result in years of progress being lost. Also, this method requires a fair bit of technical fluency, at the very least, you ought to have used some SQLite - there isn't really any other way as far as I know.
Of course, if you're certain that you want to reset the dates, I could help, but I'd have to know how technically savvy you are before.