when we can update for IOS 7 country from Sri Lanka please let us know

It is not scheduled for release until sometime this fall (September-December). Apple has not released a specific date as of yet.

When Apple announces it, later this year.

If you have iPhone 3GS, you cannot use IOS7 this fall. Only iPhone 4, iPad2 and iPod touch 5th gen or later is supported.

http://www.apple.com/ios/ios7/features/