how to clean dirt/yellowing from old document

[PixLab]

Many possibilities, I'll give a quick one as well.

Go to menu Colors > Levels, a window opens

1. Click on the Auto Input Levels button, see if you are satisfied, white enough, etc, if yes go to step 4, if not continue to step 2
   
2. Then click on that pipette (the one at the right), it's for the white
   
3. Then take a sample on the image of something that should be white (you can sample multiple time to try different zone)
   
4. Restrain the slider of the black on the levels' histogram
   
5. Play with that middle slider right-left-right-and so until you got a good contrast
   
   

   

Another quick one...

Go to Colors > Desaturate > Desaturate... a window opens (Desaturate 2 times is not an error)

1. Put the Mode Value (HSV)
   
2. Blending option in Screen Mode