Upgrade??
How do I upgrade my OS on a MacBook Pro 10.10.5 purchased in 2015?
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There's likely something here from which to choose:
• How to get old versions of macOS - Apple Support
https://support.apple.com/en-us/HT211683
While there are newer ones there, too. Choose wisely!
These are downloadable installers; they load, & then
download and build the version, in your Mac. So they
likely would follow pattern of restart, restart, etc after.
And its wise to have backups in off-Mac storage; such
as Time Machine, dedicated on their own drives, etc.