UDF files are executable files that are mainly associated with Microsoft Excel, according to FILExt, an online database of file extensions. UDF stand for "User-Defined File," which means that the file is a user-defined script that can be written or recorded. You can create, open and edit UDF files with Microsoft Excel.

Step
Right-click on the UDF file.
Step
Click on "Open with."
Step
Select the "Choose Default Program" option.
Step
Select Microsoft Excel.
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Step
Click on "Browse" only if Microsoft Excel is not on the list. Then, search for it and click on "Open."
Step
Click on "OK." Your UDF file will open.
Step
Click on "File."
Step
Click on "Save as."
Step
Select the file extension you wish to convert the UDF file to.
Step
Click on "Save." Your UDF has been edited.