How to make this narrower

If you're talking about the first system, decrease the scaling, see: https://musescore.org/en/handbook/3/page-settings#scaling.
Furthermore my recommendation for a good layout to use breaks at the end of each system, see: https://musescore.org/en/handbook/3/breaks-and-spacers.

If by "narrower" you mean, you want less space between systems, now ifs not the time to fiddle with that - any adjustments you make now will likely turn out to be counterproductive once your scores is finished. Waiting until you are closer to finished with your score between worrying about spacing. And FWIW, if you do end up with only three systems on the second page but more than that on the first, better to add page breaks to balance your pages. Then you don't end up with huge wasted space at the bottom of the page either.

If after completing your score you need more assistance getting to look its best, don't hesitate to come back here and attach the actual score rather than just a picture - than we can understand and assist better.