This is the method we usually use to removed the limited BT firmware and replace with the full Efficient/Siemens firmware on these routers.
Downloads
Config Manager v3.96 - http://www.lettronics.com/default.asp?page=lredownloads&fileid=39
5861 (1Mb) Firmware v5.3.175 - http://www.lettronics.com/default.asp?page=lredownloads&fileid=40
5861 (2Mb) Firmware v6.3.071 - http://www.lettronics.com/default.asp?page=lredownloads&fileid=41
5861 ASIC (1Mb) 15k version - http://www.lettronics.com/default.asp?page=lredownloads&fileid=43
5861 ASIC (2Mb) 50k version - http://www.lettronics.com/default.asp?page=lredownloads&fileid=42
sb5830 Firmware v6.3.071 - http://www.lettronics.com/default.asp?page=lredownloads&fileid=44
Download the Config Manager v396 from http://snake.feldtest.de (or links above) and also the latest firmware from there as well (if yours is actually badged BT, then it only has a 1Mb flash (unless its been upgraded) and cannot run the latest firmware).
If you have a 5861, download the correct ASIC.AIC from http://www.uk-bug.net (or links above) and also V5 firmware if you only have 1Mb flash can be downloaded from there as well.
Install Config Manager (assuming default location of c:\dsl for rest of this guide)
Set your PC IP to static, with address 192.168.254.100
Copy the downloaded kernel.f2k to c:\dsl, along with asic.aic if applicable (5861 only)
Load Config Manager, select Tools, then BOOTP. Fill in the details as prompted.
Turn off router, press and hold reset switch on it, and power on. After approx 10 secs, all LEDs light up briefly, release reset at this point.
This should network boot the router, and allow you to connect to it with telnet.
Type telnet 192.168.254.254 and log in.
Type the following commands.
format disk
copy tftp@192.168.254.100:kernel.f2k kernel.f2k
copy tftp@192.168.254.100:asic.aic asic.aic (only for 5861)
reboot
Once router has rebooted, you should now be able to use a web browser to access web gui http://192.168.254.254 to perform final configuration.
(we intend to enhance this guide based on feedback, so please let us know how you get on 
Get free EPoS software at http://www.lettronics.com