Query for it like you would any other element using document.querySelector
or document.querySelectorAll
.
document.querySelector("link[href='u1.css']").href = "u2.css";
Alternatively, you could also access it via document.styleSheets
as well.
As an example:
// Change [href] on first stylesheet to u2.css
document.styleSheets[0].href = "u2.css";