Sorry, but this post is way to vague to be able to provide you with a specific answer.
There are several basic ways to do a mass update of records in your database.
Replace Field Contents can modify the data in a field for every record in your found set. If you Show All Records first, this will modify every record in the table.
A looping script can loop through a set of records modifying fields in each record one record at a time.
Import Records with the Import Matching option specified, can import data into matching records, modifying only specified fields in each record with data from the source table/file from which you are importing the data. Like Replace Field Contents, this works on your current found set of records, so be careful to show all records first if you want to do this for every record in your table.
In all of these methods, the changes made cannot be undone so make a back up copy of your file before attempting any of them.