The "Killed" message usually means your process consumed too much memory, so you may simply need to add more memory to your system if possible. At the time of writing this answer, I've had to increase my virtual machine's memory to at least 768MB in order to get composer update to work in some situations.
However, if you're doing this on a live server, you shouldn't be using composer update at all. What you should instead do is:
Run composer update in a local environment (such as directly on your physical laptop/desktop, or a docker container/VM running on your laptop/desktop) where memory limitations shouldn't be as severe.
Upload or git push the composer.lock file.
Run composer install on the live server.
composer install will then read from the .lock file, fetching the exact same versions every time rather than finding the latest versions of every package. This makes your app less likely to break, and composer uses less memory.