There are many ways to run a Tiki on AWS.
If you use a stack or a script to install you are dependent of this this stack or script.
Still you could (I would do it that way) create a second instance (if needed resize temporary) install a new Tiki (the version you want) and reuse the database (duplicating is safer). You will then have to copy themes and files. Once all clear delete the previous instance.
I installed a few Tiki on AWS and as I needed to be able to control anything I installed a LAMP package with Webadmin and then...