WARNING!!!
YOU WILL LOSE PHONE'S WARRANTY
DON'T BLAME ME IF YOU SCREW UP YOUR PHONE
AND MOST IMPORTANT, DON'T PANIC IF YOU DO SOMETHING WRONG AND YOUR PHONE DON'T WANT TO TURN ON OR DISPLAYS CONFIGURATION ERROR, FOLLOW THIS STEPS CAREFULY AND REFLASH YOUR PHONE.
K550i Specs:
2G Network - GSM 850/900/1800/1900
102 x 46 x 14 mm, 85g
TFT Display capable of 256K Colours (176 x 200)
Micro M2 Slot
64MB Internal Memory
GPRS/HSCSD/EDGE
Bluetooth V2.0 with A2DP
Infrared Port
USB 2.0
MMS/SMS/Email/Instant Messaging
2MP Camera with Auto-focus and LED Light
xHTML/WAP V2.0 Browser
MP3/FM etc
When you flash it to W610:
The phone is greatly improved when the firmware is changed to its almost identical brother, the W610i. When you change it over, you will end up with the brilliant Walkman 2.0 software (with megabass etc) and fancy features such as flash menus. Basically, you turn it into a W610i with a superior camera.
Requirements:
Microsoft Windows 2000/XP (x86)
SEMC Flash Driver installed (just install the Sony Ericsson update service to get this)
XS++
W610i FS Image
W610i MAIN Image
FastPort USB Cable (the one you get with the phone)
Let's get started!!!
1.First of all, you must back up all your contacts and files, and remove your micro m2 card if applicable.
2.Turn off your phone, and remove and re-insert the battery. When done, keep holding down the C button.
3.Start XS++.
4.Click Connect
5.Keep holding down the C button, and insert the USB cable
6.If your phone was found, you should see a load of information appear
7.Now, put a tick in the box next to "Flash Main Firmware" and click the "..." button. Navigate and select the file you downloaded named "W610_R1KG001_MAIN_GENERIC_NA_RED52.MBN"
8.Now, put a tick in the box next to "Flash File System" and click the "...". Navigate and select the file you downloaded named "W610_R1KG001_FS_EUROPE_2_RED52.FBN"
9.Next, put a tick in the box named "Customize File System"
10.Select the phone model "K800", and no this is not a typo.
11.Select the CDA/Region named "EUROPE_2 (CDA102568/11)"
12.Now, you can click "Flash". Make sure the process is not interrupted, you do not have to intervene at any stage.
13.When XS++, says "Disconnected...Unplug the phone", you may unplug your phone and close XS++. DO NOT DO THIS BEFORE IT SAYS THIS, EVEN IF FLASHING APPEARS TO HAVE FINISHED.
14.Remove and Re-insert your phone battery, then turn it on. You will be able to select Normal and Flight-Mode and then you will see a screen saying "Please Wait", with a progress bar. This stage takes about 2 minutes, after that, it's done!
15.You now have a K550 running as a W610. Now you can re-insert your micro m2 card, and restore your contacts etc.
Source: Joseph's site