I assume this is in nRF5 SDK context? If so, the bootloader supports updating just the bootloader. There is no particular requirement in this case, simply make a DFU zip image that only contain a bootloader (and no SoftDevice or application). Note that if this is a BLE bootloader, then it must be compatible with the SoftDevice that is currently present on the board.
Sorry，This is in nRF5 SDK context,I want to update BLE bootloader only,How to simply make a DFU zip image that only contain a bootloader,Could you give me an example?
Yes. You generate the DFU package using nrfutil, as described in the documentation here. To only update the bootloader you include on the bootloader .hex and specifies that it depends on the SoftDevice ID that you know exists in the device.
For instance, if the SoftDevice is S132 7.0.1, the SoftDevice ID is 0xCB. Then the nrfutil command to generate the DFU zip is like this: (adjust path and name of the key and bootloader hex as needed, and also ensure that the bootloader version you specify is higher than the one currently being used):
