U-boot upgrade

Hello,
I'd like to know, wheather is there any way of upgrading u-boot different than upgrade it via running u-boot.
If new u-boot doesn't run correctly, I will not be able to downgrade the system back to previous u-boot version.
In PicoMOD1 I used stepping stone for u-boot debugging and upgrading, but there is no stepping stone in PicoCOM3. Could NBoot be used for such upgrade or is there another way?

Thank you,
Kamil

Thats what I tried, but it didn't work

I mean I tried to send 's' chars via serial line while restarting board and I obtained no NBoot response, only std. u-boot communication.
As I am not provided with any NBoot documentation, I suppose that it uses the same serial line as the one used by u-boot communication. And I also suppose that the serial line configuration is the same as u-boot's - 38400b/s 8b, 1stop bit, no parity and no flow control.

I just found out that NBoot works - its just you have to press and hold 's' key before device starts. Otherwise NBoot will not capture 's' char - its way too impatient.

I found an error in U-Boot source - file include/s3c6410.h. U-Boot version 1.3.4.

Wrong values:
#define GPBDAT_OFFSET 0x04
#define GPBPUD_OFFSET 0x08
#define GPBCONSLP_OFFSET 0x0C

should be:
#define GPBDAT_OFFSET 0x24
#define GPBPUD_OFFSET 0x28
#define GPBCONSLP_OFFSET 0x2C

Yes you're right, they are not used in standard U-Boot. However I need them in my modification