Some call the darkening process of the leather 'patina' , but i call it 'repeatedly soaked with sweat and dried'

if your concern is mainly the darkening and the hardening, then a tooth brush and some shampoo will help.
some will recommend toothpaste etc etc but i find shampoo works best, it is afterall the strongest detergent we use on our body.. and some has moisturiser too!
but make sure you try it throughly after scrubbing and rinsing (firstly press the strap in between dry towels and the best would be to keep it wraped in alot of layers of newspaper overnight to extract all the moisture trapped inside)
but if your concern includes the smell... then it will be alittle more complicated..