I set up a testing environment on a local machine and I'm using Hotposts 5.3.8 but on this VM I can't upgrade this compontent unlike all others. The download process results in a 403 error.

Maybe this machine is not allowed to download your package even if in possession of the valid licence key?

I customized the code to many parts of the Hotposts core and I really can't upgrade right on the live website, I need to be able to track what needs to restored one bit at the time