How to remotely access storage PC?

I built a PC for storage but its currently not at home, instead it's at the stepdads shop for him to use rather than him going out and spending more on storage for 3 laptops. I have access to it when I am at the shop, but remotely I know of no way to access it.

The system runs on Windows 10, both his drive & mine are two separate physical drives - these are accessed as mapped network drives. Ideally Id like to share data between this & other devices from anywhere so I don't have to go down to the shop simply to upload/download. As mentioned above I know of no way to do this and with it currently serving a business I don't want to take it offline for an extended period to change OS or set the whole file access up again.

Is there any free program that I can simply download to the machine, which once set up will give me remote access to this PC?

Apologies if this is in the wrong category, I couldn't decide to ask this under networking or storage.