A 2015 13" MBP can support latest OS 11 Big Sur, Catalina 10.15, or Mojave 10.14. I like Mojave because it still supports 32 bit apps on my 2015 MBP. I would recommend going no further that Catalina at this time. Big Sur is too new and may need some more updates to resolve bugs.
You can use macOS Recovery, your computer's built-in recovery system, to reinstall the Mac operating system. Reinstalling macOS doesn't remove your personal data.
support.apple.com
See the "macOS Mojave 10.14" link in the "Download macOS" section: