I've tried to make this foolproof and while the likelihood of unrecoverable things going wrong is extremely low, it's never zero. So proceed with care and make sure to grab a backup using Save full flash before flashing your device.
Once you start Write flash from file or Flash English firmware, you should avoid disconnecting your device or closing the tab until the operation is complete. Writing a full flash from your backup should always restore your device to its old state.
Full flash controls
These actions will allow you to take a full backup your Xteink device in order to be able to restore it in the case that anything goes wrong.
Save full flash will read your device's flash and save it as flash.bin. This will take around 25 minutes to complete. You can use that file (or someone else's) with Write full flash from file to overwrite your device's entire flash.
OTA fast flash controls
Before using this, I'd strongly recommend taking a backup of your device using Save full flash above.
Flash English/Chinese firmware will download the firmware, overwrite the backup partition with the new firmware, and swap over to using this partition (leaving your existing firmware as the new backup). This is significantly faster than a full flash write and will retain all your settings. If it goes wrong, it should be fine to run again.