Hi. I didn't uninstall SQL Server Express, if that's what you mean. I just installed PostgreSQL as well and used that. Installing Postgres doesn't affect Data Store as far as I'm aware, they co-exist side by side quite happily.
I followed the standard esri docs online about installing Postgres, I can't remember the links sorry, you'll have to search for them. I used the Postgres install files downloaded from MyEsri, but I think you can use the standard installation files if you wish, just pay attention to which Postgres version you install because I don't think the very latest one is 'officially' support by ESRI yet.