In short, the same way you installed them in the first place. In long:If you are using our site, ESOUI, you can mark the addons that you are using as favourites and then you can set it up that you can get email alerts for whenever there are new versions uploaded.
- Go to the place you downloaded them from in the first place.
- Check the addon to see if there has been an update to it.
- If yes, download it and install the update as per usual.
- If no, just keep watching to see when it gets an update by the author.
An alternative you can do, instead of manually updating them, you can download our addon manager, Minion. It's pretty straight forward, but if you need setup info check out this thread.