Set Field [ YouirTable::YOurField ; ''" ] will clear a single field on a single record.
Replace field contents on a field, with "" specified as the calculation, will clear that field in every record in your found set. Your found set can be just a few records or every record in your table.
how would I "undo" an importing of data
Neither replace field contents nor Import Records can be undone. A good precaution to take is to save a back up copy just before performing such an operation that can modify data in large numbers of records. Then, if you don't get the results you want, you can replace your messed up copy with the back up to undo what you have done.
Thanks! I used the calculation method and entered in "" and it worked!