If you want to give the leather new life without changing the historical value use Aussie to preserve it. On the other hand if you want to clean it up you could clean the leather with fresh lemon juice. Use some Aussie leather conditioner and weather proofing on it. Get some leather dye, cut it 1:1 with dye thinner, apply evenly. Buff it and apply some more Aussie.
http://www.tandyleatherfactory.com/en-usd/product/aussie-leather-conditioner-15-oz-2199-00.aspx
Test everything on the backside of the holster in a small area first.