The phrase "to tweak the design" means to make small changes or adjustments to the way something looks or functions in order to improve it. Full definition
Companies need to differentiate themselves from their competition in order to survive, whether it's tweaking the design of their products or changing the service model. (creditdonkey.com)
The stunning LED headlights return to the A8's face, with a slightly tweaked design for the new year. (cars.com)
The bill has since undergone further tweaks designed to strengthen its bipartisan support. (sciencemag.org)