Do you mean how do you upgrade a project or how do you upgrade the version?
To upgrade a project, just open it in the new version. Create a backup first. I always recommend creating the backup as a zip file. That prevents you opening it accidentally and working on it before you realise it was the backup. This way you always have a clean backup that can be used to create a new clean copy time after time. I'm not expecting you will have a problem but better safe than sorry.
To upgrade the version, either purchase at the upgrade price from Adobe or your regular supplier. Consider subscription pricing if you upgrade regularly.