Here comes the procedure:
Step 1. Hold those two buttons marked in red at the same time
then, you will enter the interface below:
Step 2. Click “System Information” once, then click the area marked in red for five to eight times in one second continuously and quickly. And then click the area marked in green once.
After that, you will enter the interface below once again.
Step 3. Hold the return key of the device for several seconds
then you will enter the interface as shown below. Click “Get System Log”, then click “SD Card Scan”.
After that, connect the device to a computer and you will find a file named “System Log”.