Outdated templates occur on upgrades when you make an edit inside a template and the parent was changed during the upgrade. It's a way to show you something has been updated in the parent style but it's keeping your changes in your child style. Simple terms you need to open the template and merge the updates with your edits.
First, try using the automatic merge system, XF has built this to help outdated templates and works well for the most part. Although on more complex outdated templates it may fail and will tell you so.
If it fails you'll need to process the template by hand. In this example we're looking at PAGE_CONTAINER, I've made a change here and it's now showing as outdated.
View custom changes:
It'll bring up an overlay, green lines are lines of code are lines in your child style which have been modified/removed in the update, or in this example a custom link we've added:
Scrolling down further we'll see more lines:
This is the result of the outdated template. A red + green line on top of each means that the line has changed since the updated. Green is what we currently have while red is what should be in the template. You can fix this by hand two ways:
No worries... it can seem very overwhelming, we provide an upgrade service. Start a ticket and we can take a look at the problem, suggest any improvements to avoid the outdated templates and help fix them. While we do this for free in most cases, if you're upgrading from an extremely old version there may potentially be a charge depending on the workload. We'll notify in the ticket.