The phrase "significant upgrade" refers to a big improvement or advancement in something. It means that changes have been made to enhance its value, performance, or quality. Full definition
As you can see, it is a fairly significant upgrade over last year's model. (zdnet.com)
The game is releasing on both PS3 and PS4, where the PS4 version will see significant upgrades in its performance but the games will have the same content. (dualpixels.com)
Significant upgrades from 1987 to 1994 increased the water treatment capacity from 27 to 60 million litres per day. (cstwastewater.com)