How do you edit transformer settings in FME Cloud?

You'll need to edit the workspace on your desktop and then re-upload to FME Cloud.

In addition to the answer above, if this is a workspace that is frequently used to reproject your datasets to different coordinate systems, you may want to look into using published parameters so all you have to do is change the parameter value (rather than editing on workbench every time).

This demo shows an example of using a published parameter to set the source and destination coordinate system (you can even narrow down the scope of which coordinate systems can be selected).

Can't do that as I only have access to FME 2016 and it doesn't support GDA2020. That's why I was using the cloud version.

Aha. Unfortunately even if you upload your old workspace to a new FME Cloud it will keep old transformer definitions. You have a few options:

1. Get your hands on FME 2021 for your desktop, edit the workspace and upload it to Cloud. This is highly recommended, especially if this is an important workspace, as support for FME 2016 is no longer available.
2. Get somebody else with FME 2021 to edit your workspace and upload it to Cloud.
3. Not sure whether this would work, probably @stewartatsafe​ or anybody else from the Cloud team can say more about this... If, and this is a big if, newer FME versions will use the latest coordinate system libraries for old workspaces you can try entering the coordsys shortname in your workspace, upload it and see what happens. There's multiple GDA2020 options though, you'll need to know exactly which.