Updating to v4 from v3 won't migrate your previous data. Only commence upgrading if you are not in production. If you are in production you have to wait for easy migration path to be released.
In order to update backend you would have to first remove the volume created for CMS assets (translation, logo etc.) so the new assets would replace the old ones. Keep a backup of volume contents if you need to.
docker volume rm root_taxiassets -f
Running above command would remove the volume. Now using below command backend & CMS would be updated to latest version.
wget -qO- http://uploads.ridy.io/docker-compose-flutter.yaml > docker-compose.yaml && docker-compose pull && docker-compose up -d
Then login into the dashboard in some cases if required to configure you will be redirected to configuration page.
Updating those will depend on changes done if any (This includes configurations you have done as instructed on their own sections). Using a VCS (Github, BitBucket & etc.) can help you track of changes and upgrade easily. It is important that you update your Android & iOS client is in line with server installation. If not in some cases unexpected errors could come up.
A good setup for VCS would be to have a master branch with base files you download from codecanyon and a branch with your own changes. You can you can rebase your version with latest version and benefit from updates easily.