How to tell android to open a file

I am trying to create an automatic update system for my game, which is currently distributed as an apk file you can download from a website. When an update is required, a popup comes up with a download button. If you press it, it downloads the file from online to the Download folder. I want to have a button come up that says "open" and if you click it, it will open the file and install it as though you went in the file explorer and clicked on it. How can I do this?