In elasticbeanstalk, can I ssh in and manually restart the (node) app?

To modify the running code, edit the files in:

/var/app/current

Then, running:

sudo pkill -f node

will cause your node app to be restarted, with the latest code running.

Elastic Beanstalk uses upstart to manage its processes.

You can use initctl to restart the processes.

For example

initctl restart nodejs

Obviously you can start it with start instead of restart if it's not already running for some reason.

Similarly you can apply the same to any process managed by upstart. You can find the names of these processes using

initctl list

See my response to a similar question in SO

The gist being, for a nodejs specific app, try

# to start the process
python /opt/elasticbeanstalk/containerfiles/ebnode.py --action start-all
# to stop the process
sudo python /opt/elasticbeanstalk/containerfiles/ebnode.py --action stop-all