All you need to do is log in using the same account that you used to download the apps.
You can update via iTunes or the iPhone itself if you are connected to the Internet.
To update via the iPhone. Click the blue icon on the homescreen, inside it has a 'A' and is labelled app store. You will then need to press the far right tab at he bottom, Updates. It will check for updates then if any are available it will come up with a list and you will need to choose which ones. You will then be prompted to login using the SAME ID that you used to dowload it.
Alternatively open up your iTunes and press applications from the far left menu. In the bottom right there is an update button. Then continue as prompted by the options and as said above.