You can't update data on an existing field unless you use a 3rd party application.
I used a method of segmenting the projects that I wanted to update with a field and adding them to a filter in a report. So, if 200 projects were being updated with abc data, only those 200 projects will appear in my report. Then that allowed me to 'mass select' the projects to update like that. See the comment I pasted below - it wasn't pretty but it was functional.
Hey Dawnmarie. I had to backfill a number of projects with a new custom field. Since these projects existed already, I couldn't use a kickstart.
I did find a workaround however, although crude. I created a report and filtered on project # equals xyz or project # equals xyz. I then switched over to text mode and copied the code.
I pasted into excel and used =CONCATENATE to join the first line and project # (user ID in your case). Each new record was a new line. I added an equal number of second lines beneath these records. Then I added a number order to each area. (so each section had lines labeled 1-25). When I chose to sort by that number, it puts them in number order.
Once those are all segmented in a report, you can mass edit as needed.