Actually, I have no idea. Good question.
Any time I hit a glitch (three-four tmes an hour) I call the Upwork helpline. Thiose guys are terrific. I tell them if I drop my pencil under my desk, I will call them. They're pretty good.
No need to call customer service. There are many freelancers who are also clients and are able to do this easily. The information you need can be found with a google search. I did one and came across this article which answers your question: