There MUST be a way to update records in bulk. But how?

There MUST be a way to update records in bulk. But how?

I have created a database with thousands of records.  Occasionally, I need to add another field, after which I need to populate those fields for each of the records.  I do NOT want to go through each record one by one to update it.  The information I need is in a spreadsheet, which can be converted to CSV or TXT file with no problem.  The information is numerical, therefore the bulk update option does not work, as that allows you to update records only with the same value.

Most databases allow you to change records with a bulk upload, but I have not yet figured out how to do this.  Any upload creates new records, which is unworkable because then I have duplicates.  

Does anybody know how to accomplish this?