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There are two techniques. The first technique is to convert the field from what ever type (i.e. numeric, text, date) it is to a formula field of the same type (i.e. formula-numeric, formula-text, formula-date). Then convert it back to the type it was. This erases the contents of the field. To learn how to change the type of a field please read the following:
The other method involves creating a custom report with all records and two columns. The two columns should be the table's key field and the field that you want to erase. When you've displayed this report click on "Other" and then select "I want to export this view to a spreadsheet". Within the spreadsheet erase the contents of the column you want to erase without erasing the column label at the top of the column. Then select both columns, including the field names at the top of the columns and copy them to the clipboard.
Follow the instructions for copying and pasting from another program in the following help topic:
The columns should automatically map back to the columns that they came from. After you complete the import the column's contents will be erased.