If you need to update but it won't do it automatically, open the launcher and go to "Options" next to your username. Now click "Force update!" and it should say "Will force!". Now just log in and it will update to the latest version. Just remember that it will clean out all your mods, though.
What shall i do?
it wont force update either
If you haven't downloaded the game, and it's not installed as a program, then you can't really update it, can you?