To update the value of a UDF on an existing project use the steps given here:
 
1. Click on the name of the project in the 'List Project' screen.
2. Click on the project name link displayed in the "Project Structure" section.
3. On the "Edit Project" screen update the value of the UDF and save the project.  Check the print-screen "Update UDF value on project.PNG" attached to the solution.
 
If you would like to update this for multiple projects then you may do a mass edit for projects.
-Follow the steps given here to do a mass edit for projects in the following knowledgebase article:
 
How to Mass Edit Projects?