Does anyone know how to replace 9z built in lithium battery, I have opened the back cover but did not find any objects look like a lithium battery.

pls send me pictures if you have

Hi pentras,

We here at the Futaba Service Center cannot give you instructions or help in changing out your lithium batter. We can only suggest that you send this into our service center to have our technicians check this out for you.

I hope you find this information helpful. You can also reach our Futaba support technical team at 217-398-8970, or via fax at 217-398-7721.

Sincerely,
Krysta
Lead Futaba Customer Service and Programming Technician

I just replaced mine but that's not easy as there's a lot of cables to remove, together with some PCBs.
The lithium battery is located under you battery compartment and to remove it, you have to open pretty much everything. You want to take your time, work by steps and use different small cups to store the screws for each step. You then want to remove the board and unsolder the old battery using the correct tools for the job.
If you're not familiar with electronics, I would not recommend it. Send it to a repair shop.
Otherwise, I'll be in Hong-Kong on the 19th of September for a week! ;&gt

my 9z was brought in Hong Kong, can I send it to Futaba Service Center USA for repair or else?

If you're in Hong Kong, there are service centers there. You can locate the distributors at:

http://www.futaba-rc.com/futdist.html

There are several in Hong Kong.