I tried to key "B" on terminal tab of X-CTU and hit Enter .But no reaction on the screen. then i tried to key "+++", it's the same result! so how to use X-CTU update the bootloader of XBee S2
so how to use X-CTU update the bootloader of XBee S2
You can not replace the bootloader on the XBee. Only digi.com can do that.
You can update the XBee module "settings" using a standard XBee carrier board, and XCTU.
You can update the "firmware" on XBee modules [at least S1, I've never tried S2/etc], but you need a special carrier board to do this, or else modify a standard XBee carrier board. XCTU needs to be able to control the DTR, Reset, CTS, or some other pins on the module. You have to research this.
I'm not sure what XBee "bootloader" means.