You can search for "macOS" in the App Store to get the latest version of macOS 11.2.3 Big Sur. Or you can find links to other versions of macOS from 10.13 to 10.15 within this Apple article which also includes directions for creating a bootable macOS USB installer:
https://support.apple.com/en-us/HT201372
Another option is to boot into Internet Recovery Mode using Command + Option + R to try to boot to the online version of the macOS 11.2.3 Big Sur installer.
I highly recommend you have a good backup before you attempt to upgrade macOS.
You should always have frequent & regular backups since it is impossible to recover accidentally deleted data from an SSD after the Trash has been emptied plus an SSD can fail at any time without any warning signs.