I would have thought this would be a question I could find the answer to with a quick Google search, but I can't. I have connected to my EC2 instance via an SSH client but now I would like to disconnect, but I have no idea how.
Simply typing the exit command will log you out of your instance.
Hope this answers your question.
There are a number of ways you can end your SSH session.
Simply closing the client should be sufficient.
You can type exit or logout to explicitly end your session.
Another approach is to use Ctrl+D which will send an EOF to the shell and terminate your session.