libcurl/url/set-part.md: show how to update a URL part

Closes curl#253

libcurl/url/set-part.md

@@ -20,3 +20,37 @@ provided as a string this way.
 
 Set parts are not URL encoded unless the user asks for it with the
 `CURLU_URLENCODE` flag in the forth argument.
+
+## Update parts
+
+By setting an individual part, you can for example first set a full URL, then
+update a single component of that URL and then extract the updated version of
+that URL.
+
+For example, let's say we have this URL
+
+    const char *url="http://joe:7Hbz@example.com:8080/images?id=5445#footer";
+
+and we want change the host in that URL to instead become `example.net`, it
+could be done like this:
+
+    CURLU *h = curl_url();
+    rc = curl_url_set(h, CURLUPART_URL, url, 0);
+
+Then change the host name part:
+
+    rc = curl_url_set(h, CURLUPART_HOST, "example.net", 0);
+
+and this then now holds this URL:
+
+    http://joe:7Hbz@example.net:8080/images?id=5445#footer
+
+If you then continue and change the path part to `/foo` like this:
+
+    rc = curl_url_set(h, CURLUPART_PATH, "/foo", 0);
+
+and the URL handle now holds this URL:
+
+    http://joe:7Hbz@example.net:8080/foo?id=5445#footer
+
+etc...