As you are asking this in a Java forum, I assume you have a Java program that reads from a database, allows the user to change the values and then saves the data back to the database. Without more details of your program it is difficult to give you more than general advice such as you will need to have flags of some sort that indicate whether the data has been changed or not and then only save data when these flags indicate that it has changed.
In my application i have a java program to display more than 300 fields in the screen.If i update all the fields the system speed becomes slow so what i want is to identify the fields whose values are modified and update those modified fields alone to the database.
So what is exactly the question, please be more specific. Identifying which fields have changed shouldn't be too difficult - you'll have to save the original values somewhere at the beginning, and when the time comes to update the data in the database, you compare the current values with what you had before. If the value is different, then you obviously have to update it in the database.
If you want a more specific answer, then post a more specific question. What exactly is your program doing now (post some code if necessary) and which part of it exactly runs slowly?