Your AWS instance has an SSH server already configured and running, so you can access the machine to manage it from the inside.
You tried to update from the Mautic Dashboard but you got an error. It is most likely to be something related to permissions. We configure the application with the most restrictive permissions that still allow the application to work correctly so your system is as secure as possible. That's the reason why an update via command line may work and also it will be easier to debug.
You can try to follow our official documentation (the links I posted previously) as they should be ready for non technical people. Ask us any questions you may have regarding steps found in the documentation.