Here is a guide to changing the housing on the Sony Ericsson K850i phones.
1) At the back of the phone towards the bottom, underneath the battery cover locate the two screws which are in either corner.
2) Undo these screws using the Torx screwdriver (either T5/T6 size is ok).
3) Once you have undone the screws, the back cover can be removed. You will need a slim plastic shim or credit card. Run it underneath the back of the phone just to unprise it. The back cover will then be able to removed.
4) Locate the screws on the back of the phone near the top. Again, they should be in each corner which hold the front cover onto the phone.
5) Undo the screws using the screwdriver.
6) Similar to the back, the front now can be removed.
7) Replacing the housing is just a reverse of these steps. Take care when replacing the front, making sure that the keypad does not get caught in the frame.