Hi,
This post:
has a link to “Downgrading from Enterprise Edition to Community Edtion” which you can follow through the steps. You have to disable EE functionality first, and then once that has done, you can install the gitlab-ce omnibus package or build CE from source, depending on which version you chose when you installed EE.
The link explains the steps you need to take to do this and what commands to run to disable EE functionality.
Obviously make sure you have a backup of your server before you continue, so that you can restore it if it doesn’t work.