You could create an Opus button which automates it using a command-line tool like ExifTool. With that you'd select the files you want to change and click the button to make the change.
Assuming we're talking about JPEG files, and based on this thread:
It looks like the command in Opus would be like this:
"C:\Program Files\ExifTool\ExifTool.exe" -jfif:Xresolution=300 -jfif:Yresolution=300 {filepath$}
Or possibly:
"C:\Program Files\ExifTool\ExifTool.exe" -Xresolution=300 -Yresolution=300 {filepath$}
Changing the two 300 in each command to the DPI you want, of course.
ImageMagick is another command-line tool which can do this, but it seems to want to re-compress the JPEG data, which isn't good if you just want to edit the DPI tag. (Maybe there is a way to tell it not to, but using ExifTool seems easier.)