Hello Nattapong,
To change your Steam account email address, please login to your Steam account and follow the instructions below:
Click on 'Steam' in the upper left, then select 'Settings' ('Preferences' for the Mac client).
Select "Change Contact Email Address..."
Enter your Steam Account password along with your new email and click next.
Check your new email and enter the code sent to you by Steam Support in the dialog box, and click Next. Once you have entered the correct code, you can click 'Finish' to complete the process.
Please also ensure that your email is verified once you have updated your account.
Title: E-Mail Verification
Link: http://support.steampowered.com/kb_article.php?ref=5151-RUAS-1543
Please let us know if you have further questions or concerns.