Q: How to unlock iphone 5s, purchased from US, and want to use in saudi arabia, it is lock for AT & T.

You need to contact AT&T to unlock the phone.  They are the only ones that can do it.

http://support.apple.com/kb/HT5014

Contact AT+T - only they can unlock it, if you qualify. See: http://www.att.com/esupport/article.jsp?sid=KB414532&cv=820

You would need to ask them.  There should be no cost (if your account with them is in good standing).

Then you have a problem then - you need to get the person who held the account to contact AT&T and they will probably only unlock it for them.  If you are not an account holder then AT&T will probably charge you for unlocking the phone.

Please read and understand the reply from Ethmoid above.