Cannot install Krypton

I’ve downloaded Kyypton using the ARM selection. When I go to file manager and try to open the file for install , I only have 2 options..Package Installer or an anti-virus app. When I choose either one, I get the “app not installed” message. How can I change this to open with a web browser, or in a way that it will install? I have Chrome installed.

Please keep in mind that I am far fro a computer expert. My Android operating system reads is 5.1.1 API level 22 (Kernel:Linux 3.14.29)

I’ve spent a ton of time trying to figure it out, all to no avail.

Thank you.