its pretty easy
1.. download ODIN on ur PC
2.. attach data cable with PC
3.. backup all ur data and apps from your current ROM with the help of any app ( if you arte already rooted then use mybackup root from market ) its very easy just install and run and select BACKUP and backup all your apps first and then your data
4..note down your MIN no just in case
5.. switch off phone
6.. remove battery
7.. hold vol down button and insert data cable
8.. a big yellow DOWNLOAD triangle will show on phone screen
9.. double click ODIN and run
10.. you ll see a yellow COM port number flashing in one of the top empty boxes , that means your phone is connected to PC correctly and with ODIN as well
11. now dont touch any other option on ODIN screen just hit PDA button mind it just the PDA button and give path to the EH03 odin TAR file stored on your PC
12.. hit start and leave the PC and phone alone and dont touch anything
the process will take 5 to 10 min , you might feel its stalled somewhere but dont touch anything let it finish
13.. once finished the screen will go blank on the screen and in ODIN you ll see a message of SUCCESS
14.. put battery back and boot ( booting for the first time will take longer )
have fun !!!