upgrading a firmware for swann DVR via serial

[westhealth]

I am trying to upgrade firmware on my current Swann DVR using UART, my previous swann DVR gives me an option to stop the boot(ctrl+U) like this

U-Boot 2010.06-svn (Nov 24 2015 - 16:37:38)[V1.2.1]
*** Warning - bad CRC, using default environment
Hit ctrl+u to stop autoboot: 0

but my current unit doesn't give me an option to stop the boot sequence like my previous one, how can I upgrade? thank you
my current one goes like this

U-Boot 2010.06 (Mar 02 2016 - 10:50:25)

Check Flash Memory Controller v100 ... Found
SPI Nor(cs 0) ID: 0xc2 0x20 0x18
Block:64KB Chip:16MB Name:"MX25L128XX"
SPI Nor total size: 16MB
In: serial
Out: serial
Err: serial
16384 KiB hi_fmc at 0:0 is now current device

UpgradeSoftware:0xffffffff
CheckUpgrade invalid addr(0xffffffff)
uboot compile time: Mar 2 2016, 10:50:38
16384 KiB hi_fmc at 0:0 is now current device

16384 KiB hi_fmc at 0:0 is now current device

jpeg decoding ...
<<addr=0x9fb00000, size=0x10000, vobuf=0x9fb20000>>
The logo picture existing!
mmu_enable
PicType: 6 ,Output Addr, Y: 9fb20000,UV: 9fb8c000
<<imgwidth=720, imgheight=576, linebytes=768>>
decode success!!!!
dev 0 set background color!
OpenDev vo:0 intf_type:36 out_sync:17
dev 0 opened!
layer:0 , addr:9fb20000 , stride:768 , (152,96,720,576)
video layer 0 opened!
The key_id is 3 .
16384 KiB hi_fmc at 0:0 is now current device

## Booting kernel from Legacy Image at 82000000 ...
Image Name: Linux-3.10.0
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2887560 Bytes = 2.8 MiB
Load Address: 80008000
Entry Point: 80008000
Loading Kernel Image ... OK
OK

Starting kernel ...

 

[alastairstevenson]

my current unit doesn't give me an option to stop the boot sequence like my previous one, how can I upgrade?

Try other characters such as Control-C or * or <RETURN> and more.
What works does vary with the model.