Firstly, I'd recommend backing up your scores by exporting your lead data to a CSV (or multiple if the db is quite large). Then you'll have something you can revert to.
If you want to reset all scores to zero, this is straightforward: Simply make a batch Smart Campaign which sets all score fields to zero or NULL.
If you want to maintain some of the existing scoring, you can make the aforementioned Smart Campaign smarter by resetting to specific score values based on what score they are at.
e.g. If Lead Score > 200: Change Score to 20
If Lead Score is between 100 and 199: Change Score to 10
Hope that helps!