I believe you are referring to the Upper Gallery. In the US, this uppermost balcony of a theater would usually be called the upper gallery. Upper balcony is also understandable.
However, the name of this section varies according to the theater. Is there a map of the theater online where you can check to see what the proper name is? Ticketmaster generally has maps of venues for their ticketed events. The theater website might also have a map. If you cannot check online, you could call the theater and check. Good luck!