When you have a website built, it should be done using the latest code available to ensure that it is secure and functions correctly on the newest browsers. Over time this code will become dated and you will need to ensure that updates are carried out to keep it secure and to ensure that it works how it should. Depending on what content management system you sue, you may be able to get the updates to be done automatically. For example, WordPress roll out updates every month or so which you can choose to download. Along with being able to download the latest Wordpress updates you can also choose to update the various plugins that you use.
If the content management system you use is bespoke, then it will need to be updated manually. This can be harder work but is often required for more bespoke websites where an off the shelf content management system is not suffice.
Make sure you always take a full back up of your website before doing any major updates and ideally test them in a staging area before doing them on the live site as this will allow you to find out if any issues may be caused by doing the invites.