upgrading from Yosemite to High Sierra
How can I upgrade form Yosemite to High Sierra on 2015 MacBookPro?
MacBook Pro 15", macOS 10.14
Click on this link and go to Section 4, How to upgrade to macOS High Sierra - Apple Support
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
Check that your printers/ scanners etc have updated drivers and software available for the new OS.