If you make a change to the code, you hold the copyright to your changes. You are therefore entitled to add your own copyright notice. The copyright for the file with your changes is then shared between you and the original authors.
In my experience, copyright notices are often ordered newest-to-oldest so that the newest notice is right at the top of the file. But there is no requirement to do so and you can of course put your notice below the others.
Some projects don't want to add a copyright notice for everyone who contributed to that file. Instead, some notices only attribute the original author (with the implicit understanding that other people might also hold part of the copyright), or attribute the project contributors collectively. If you want to contribute back to such a project, it would not be appropriate to add another notice, even if you are allowed to.