Finally I can update my Samsung Galaxy 5 (GT-I5503) with newest firmware. No, the update process is as simple as click and go. The main problem is how to enter Download Mode for the device.
Almost on all tutorials that I follow (one of them), state this:
Hold “Volume Down, Home button & Power button” in the same time to enter Download Mode
With GT-I5503, you will get nothing. Android will starts normally.
After explore (and register) on many forums, I got insight from one of forum member, that he did it by pressing OK button, rather than Home button. So I give it a try, change the combination to “Volume Down + Ok Button + Power Button” and voila, yellow Android logo is there, on download mode.
Hope this will help someone that lost like me.