Step template changes are currently in the audit log. The events look like "ActionTemplate was modified". You can filter the audit view by "Document Created", "Document Modified" and Document Deleted" to help find these events. In our next release or two we are adding additional filtering which will allow you to filter just by document type (such as ActionTemplate). We are also adding the ability to export the filtered log to csv.
In the mean time you could query the Octopus database directly with a query like:
select * from dbo.[event]
where RelatedDocumentIds like '%actiontemplates%'
order by occurred desc