How to update MCU bootloader?
It is possible to remotely update a bootloader, but you have to be careful, because there are certain points in the process at which a power failure or other error could "brick" the system.

The general strategy is to use the existing bootloader to replace the normal application code for the product with a special "bootloader update" application. This application contains the new bootloader code, along with enough firmware to write that code to the bootloader area of the device and to verify it. Once the new bootloader is verified, the special application allows it to run on the next system startup, at which time, you can use the new bootloader to write the normal application code to the device.