One of the notable additions is “untrusted” devices (where the sync data and file names are encrypted on a remote device, but still properly synchronized), improvements in handling conflicts, improvements in the WebGUI (for example: direct link to edit ignore patterns from the folder status)…
To try an updated version, backup the config.xml, cert.pem and key.pem (you can also backup the https-cert.pem and https-key.pem, but those are only for the TLS connection to the WebGUI). See Backing and restoring a "primary" Syncthing node and How to save configuration of Syncthing.
Do the upgrade on both sides – many of the benefits won’t be seen unless both sides talk (close to) the same version.
If you don’t like what you get, replace the files you backed up on both sides and delete the database directory, then restart with the old version. The database will be rebuilt and it will look like the two sides are exchanging a lot of files, but if they were already in sync all you are seeing is the exchange of metadata as the two sides catch up.
No risk at all.
And do let us know how the upgrade goes, its an interesting data point to validate the developers’ efforts at maintaining continuity (a very laudable effort, which I am sure costs no small amount of frustration at times!).