Please go to My Site → Customize → CSS (or Additional CSS), then add all of the following code to the bottom of that window.
/* This CSS hides the WP.com footer credit as there is a bug in the theme keeping it from changing.
* 11236016-hc (jw)
*/
.site-info {
display: none;
}
Tip: keep the /*
comment */
in the code to later know what this code does.