This is an amateur photo of my sister - how would I edit out the armpit sweat stains from walking around all day?
image cropped to save space - original available on click-through
Colour-dodge will do it.
You didn't say what software you have access to, though I assume most editing software will have a Dodge/Burn feature.
In Photoshop I set it to dodge shadows with about 40% strength.
This was a very quick attempt, you could do better, taking more care, on the original full-sized image...
By the way dodge and burn technique was invented in the film’s darkrooms when you where enlarging negatives to positive copies. Working with negatives the more time the light from the enlarger hits the “shade of” white photosensitive paper the darker it will get after finishing to develop it.
So to dodge consists of putting something (a black mask) between the enlarger and the paper over the part that you want lighter in the the print (dodge = escape, hide) so it is exposed to the enlarger light less than the rest of the print.
And to burn means the contrary, exposing a section of the print more time to the light of the enlarger to make it darker. It is actually done by masking the rest of the print (a dodge to the rest).
But today, we work with digital positives, there are no native negative pictures. Using the darkroom terms in their literal meaning, only induces to confusion. The terms should be inverted and burning should mean to increase the exposure/brightness of a part of the picture and dodging reducing it. It would make more sense in Lightroom(the opposite of a darkroom)/photoshop or any other editing program.