Samsung's rollout of One UI 7 based on Android 15 continues, and today it's starting to reach Galaxy A35 units in Europe, after first having been released in Korea a couple of weeks or so ago.
In Europe, the update arrives over-the-air as a 3.6GB download, so make sure you grab it while connected to a decent enough network. Once you've installed it, you'll be on software version A356BXXU4BYDC. This update comes with the April 1, 2025 security patch level - which obviously isn't current anymore, hopefully Samsung will catch up soon.
If you're in Europe and haven't yet received the update notification, you can always manually check for the update by going to Settings > Software update > Download and install.