May I measure 360v or not?
Not.. 
sorry.
I did not believe that device has 360v because the input voltage is 4.2v.
The device generates high voltage to electrocute the mosquitos. Believe it or not... I guess you have seen the sparks when a mosquito gets in... A spark of 1mm takes around 1000V...
Yes. But when you touch with fingers it just shake a bit (my spark you can see in the first picture).
I found spec. of my multimeter. It's says 250+_ . That is not intended for over 250v?
Guess...
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No. It says .../250V ± (0.5% +2). That is not 250V+, the upper limit is 250V
See post #9
Oh, Damn.
Is the multimeter showing when one of the two fuses is burned? How to know? I measured like 3-4 times in different days and it's still measured. But I will not try again.
Btw, today I received a mosquito recket that says that charge is 4 hours, and I'm still charging like 6-7 h and still have a red light with no turn OFF And for it, it clearly says it is 3000v and a battery of 14500 mAh (not believe haha). Not opened the battery yet, but it seems that will be overcharged too.
Why showing me this APO? 
That is always. When there is no measurement of a few minutes, it turns OFF byself to save a battery.
An "APO" indicator means there is an option to disable it. A sticker would suffice. Most technicians turn meters off from good habits... like ESD, no rings, no dangling clothing (lost on modern amateurs), and do not need to be reminded of an automatic setting taking up valuable display space that could have been used for another digit.
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The fuses are for over current protection and I sure that the mosquito killer could produce enough current to blow a fuse. However applying 3000V could have done some damage but if there is some protection inside maybe not.
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Well, for this particular lamp, not really 3000v. It seems that is a series where the manufacturer hidden that data. Only one I found 3W says that is 1000v. Mine is 5W. And I measured (somehow) 360v.
Many chinese certificates worth nothing and 5v 3w can be converted to 5000v.
Nice round number. Can be max your 250V rated DMM outputs, even if you give it some kV.
Ok, I was taking photos of PCB for you.
Maybe you see one chip strange, that was my repair (first day new lamp fell me to the ground and loose LEDs).
We are looking if we can change something on the PCB like a charge controller. It will be perfect if just need to replace this big red capacitor 
Why?
That capacitor is just a buffer cap for the rectified voltage of the transformer.
It holds a certain amount of high voltage charge to kill an insect quicker.
Educated guess is that the high voltage winding of the transformer is isolated from the rest of the circuit. At least from looking at the bottom of the circuit board.
Battery voltage is a worry though. I only see a diode in the USB/battery area.
I wouldn’t use this device, connected to a LiPo cell inside.
Leo..
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Guys. What if I buy a USB charging module with cut-off protection like in this video and ignore (or remove) the USB port from the board?
Well, now we have to wonder if it is really 4.5V since you may have damaged your meter(s) when you tried to measure 3000V or 1000V with it.
For measure the voltage, I didn't break the multimeter. All 4 devices measure similarly the same battery. The exception may be this last digital meter, which is manually adjustable.
Edit: I checked fuses and they are OK 
Have no idea what that guy is doing. What is he upgrading from?
What was the black square thing with the red and black wires?
The black square with wires is just an old battery that comes with a recket. He replaced it with an 18650 battery with a new charge module.
That is a very popular mode in mosquito recket devices (many videos exist).
Also, on black multimeter says 750v AC,1000v DC, and max 500v, unfused.
On the red device it says max 500v, fused.
So, if is 360v my measurement, I can try measuring with one of these (they are cheap). Maybe fused better. I really want to compare my mosquito recket killer with a lamp. It seems that recket has a stronger spark!