How can I change the header size in Grid Market from 980 x 100 to 960 x 324?

Our existing website (flyinggoatfarm.com) has a header that is 960 x 324. We would like to transition to Grid Market to improve our store. Unfortunately Grid Market was written for a header that is 980 x 100. Is there anyway to change either the parent theme frame market and or the child theme grid market to work with our existing header? I'm new to Word Press and would appreciate detailed instructions. Thanks in advance.