Depends on how you have things set up.
Why not just create a new record?
As Philip said, you have not given us enough information. For example, we don't know if the "30 lines or information" come from related records or come from one big text field with 30 carriage returns. But whatever you do, make a back up copy of your current file first. You do not want to delete a year's worth of records only to realize later that you actually need some or all of the deleted records.
It sounds like it's just using Table View so what you are actually doing is by clearing all the 'cells' is editing all fields in 300 records.
In Table view each row is a record so rather than deleting them you are just copying over them with the new information.
If that's the case, and you need to retain the existing records but delete all the data in the fields (why not just delete all records or truncate table?), then you could replace field contents with an "" empty value.