Upgrading is very simple via composer, just go to the command line in the root of your project and type:
composer update
If you downloaded the zip file instead you will need to download a new copy and overwrite your existing files. It would be a good idea to export the theme you are using from the admin if you have made any changes.
If there has been a major versin update, ie. from 5.3 to 5.4 you will need to run this command first:
composer require web-feet/coasterframework:5.4.* laravel/framework:5.4.*
There may be breaking changes in these versions if you have any custom code or themes.
The routes have been seperated out into a new service provider: CoasterCms\Providers\CoasterRoutesProvider::class
You will need to add this to your config/app.php file, usually best at the end as it included some catch all routes.
Also in this update a big change was PageBuilder::block() only returns a string now. If you where using any data functions they will now need to be changed to PageBuilder::blockData()