If you need to modify the value, why not edit the HTML directly?
Or if you mean that you want to edit the value at some point after the page has loaded, after a specific action has taken place (such as after clicking a specific button), please tell us what/how exactly. JavaScript would be the way to go but we need to know more about what you’re working with/doing in order to offer suggestions. Or maybe you just need to Google “javascript change value of select box”? It’s hard for me to guess without all the details.
Also, a small CodePen demo would be more useful for us, so that we can edit it live (with JavaScript or CSS). We can’t try much out on a live site, and the full codebase tends to put potential helpers off.
