How to change from a single entry visa to a double entry visa?

You can't change your visa from single entry to double entries. However, if you fly from New Zealand to Beijing, and then fly from Beijing to Macau, you may use the 144h visa-free transit in Beijing. Then you can save the single entry for the trip from Macau back to Beijing.

Hi Sally, thanks for your reply. My trip is from NZ to Shanghai for 5 days then fly to Macau and then to Beijing. So I can use the 144 days free visa from Shanghai to Macau and then use the 1 entry visa from Macau to Beijing? Thanks

I hope you see this in time.... the visa free period is for HOURS not DAYS!
Wishing you a good trip.

Also, be VERY careful that the single entry visa does not get stamped by mistake when you enter the first time as it will be very difficult, if not impossible ,to change. Do NOT hand your passport over until you are certain that the officer understands this as they are VERY fast to stamp and it will be too late afterwards. If worst come to worst, you can probably get another visa in Macau, or failing that, possibly in Hong Kong. I suggest you check with the Chinese Embassy in NZ before you go, if you haven't already left.