Im having Gitlab CE 13.3.5 version running on ubuntu and would like to upgrade to EE. could you please guide me how can I upgrade it with existing data, what are things i have to consider before upgrade, like backup and more.
As per the docs, they list everything you need to look at doing for the upgrade process:
Backups can only be restored to the version that is installed. So backups you make before the upgrade can only be restored for the exact same version of gitlab-ce. They cannot be restored later in gitlab-ee. If it is a VM, then you can also make snapshots before installation to make it quicker to revert in the event of problems.