Well we can only show the icons supported by fontawesome.
So for changing the icon first go to
http://fontawesome.io/icon/ and search for your icon that you need.
For example lets say we want to use weibo icon then search the weibo and copy its unicode, refer:
http://prntscr.com/c8v954
Now add the following css in your appearance->customize->custom css:
a.twitter i:before {content:"\f18a";}
Where .twitter is the class which will be different if you choose some other icon. And the f18a is the unicode for the weibo icon.
Now the twitter icon will be changed to weibo icon.
Similarly you can change any icon you want, but make sure wplms can show only the icons supported by fontawesome.