You can't set the href directly like that, because document.getElementsByTagName returns all the <link> tags (as a NodeList). If you're positive you only have one, use this:

var findlink = document.getElementsByTagName(link);

findlink[0].href = stylesheetxhtml.css;

If you have multiple <link> elements and want to target a specific one, give it an id and use document.getElementById:

var findlink = document.getElementsById(myLinkId);

findlink.href = stylesheetxhtml.css;

Finally, if you want to create a new <link> element, use document.createElement:

var newLink = document.createElement('link');

newLink.href = stylesheetxhtml.css;

document.getElementsByTagName(head)[0].appendChild(newlink);