The only way to get a newer version of Safari is to upgrade your macs operating system.
A 2012 MacBook can upgrade as far as macOS Catalina, other slightly older OS's are still available too.
macOS Mojave 10.14 and macOS Catalina 10.15 run the latest version of Safari ver 14.1.1.
Although it is possible to upgrade directly to Mojave or Catalina from Yosemite I would recommend
upgrading to macOS High Sierra 10.13 first, this will prepare your mac for the new APFS file system.
For older OS’s such as High Sierra, Mojave and Catalina,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to Check Compatibility to see if your mac qualifies for the OS upgrade you want.
Then go to the Download OS section and click on the OS you want to install.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you are considering upgrading to Catalina be warned does not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are offering 64bit updates.