"Major refresh" refers to a significant overhaul or update of something, making significant changes to improve it or bring it up to date. Full definition
The last major refresh for the trucks occurred in 2007. (cars.com)
Amazon's last major refresh of the Kindle brand, both for its electronic ink e-readers and colorized Kindle Fire tablets, was last September. (zdnet.com)
This was the first major refresh for UI of Windows Operating System after Windows 8's Metro Design. (thewindowsclub.com)