I would like to know what changes were implemented in firmware 4.2 for the Flint router? Has there been an update to the OpenWRT version?
It is possible to install the beta version keeping the settings of the previous version, 4.1.0?
I apologize for the English, but I’m using Google translator
Stable and Beta builds usually come with release notes. You first need to download the beta firmware and go to upgrade in the GUI. Next do local upgrade and upload click release notes. You do not need to install. Snapshot usually does not have release notes
openwrt-ax1800-4.2.0-0214-1676344231.tar
SHA256 00a897601a1ac6c25aa70ad1bb27857f9f38da2fb3ba10e18c19bb9d34bc3480
V4.2.0
New feature
Added Parental Control feature.
Added Zerotier feature.
Added Tailscale feature.
Added support for FTP and NFS protocol in network storage.
Added Clear Traffic Statistics button for in client page.
Added DHCP Gateway option for LAN.
Added SSID Visibility option for guest Wi-Fi.
Added support for GoodCloud alerts when new clients join
Added support for comments for domain and IP profiles in VPN policy.
Optimization
Improved IPv6 LAN Mode options. Separates the native and passthrough modes.
Improved the results and hints of the DDNS test.
Improved drop-in gateway feature with DHCP-based solution to increase stability.
Improved LED lighting logic to ensure consistency with the UI.
Improved interaction for MAC address cloning.
Improved networking status alerts in Internet page.
Improved interface tracking settings description for Multi-WAN.
Improved login page with automatic focus to password input box.
Improved VPN client configuration file view with files sorted by name.
Improved the switch name in the VPN global options for whether the GL.iNet service uses VPN or not.
Improved the interaction of set read-only users to read-write users in network storage.
Improved ADGuard Home feature to support seeing which client the request is coming from.
Language
Added German language.
I do not work for and I am not directly associated with GL.iNet 
It would be nice if they posted what SECURITY Updates have been done since 4.10 once they decide to call 4.20 Stable for the Flint. Personally I don’t need any of those added remote services features (Which I view just as added vulnerabilites)…But it would be nice to see if any SECURITY upgrades/fixes have been applied since 4.10.
Yes.
The Openwrt version is not updated. But fix various bugs and setting failures accumulated from 3 months ago.