If you want to update the Apps you have downloaded from iphone store , then its a very easy process. Whenever there is a new version of a App, the phone automatically finds the same and shows you the update on the icon with the total number of Apps to be updated.
Just click on the App Store, then click on the Updates link at the bottom right end, where you will find the list of all the updates required.
Following this you will be asked to enter the logins of your iTunes account which you then authenticate your account and go for an update.
The update would be done within seconds or else minutes based on the updates size and it also depends on the connection speed. Its always recommended to do these updates when you are on a wi-fi internet access because of a high speed connectivity.