Recovering U-BOOT

Hello,

i updated u-boot with wrong file.
Now i get alway following message on serial port:

Trying Uart
BootROM - 1.73
Booting from SPI flash
BootROM: Bad header at offset D4000000
BootROM: Bad header at offset D4200000
BootROM: Bad header at offset D4400000
BootROM: Bad header at offset D4600000
BootROM: Bad header at offset D4800000
BootROM: Bad header at offset D4A00000
BootROM: Bad header at offset D4C00000
BootROM: Bad header at offset D4E00000

I tried to recover u-boot with kwboot

kwboot -t -B 115200 /dev/ttyUSB0 -b u-boot-spl.kwb

Router reboot all 2-3 minutes. I think there is a watchdog.
kwboot runs only to 20-23% in this time.

How can i recover u-boot now ?

Thanks in advance
Michael

After many replies it works with kwboot from u-boot-tools of Linux distribution Debian SID.