As far as I can tell, it isn't possible to change the Trigger step for a different Trigger.
Often we build a flow, then find the intial condition isn't correct (we need a SharePoint created or modified, not just created, for example. Or we need to switch a flow from SharePoint to OneDrive.) If it's a sophisticated Flow then rebulding it from the top is incredibly tedious and prone to mistakes.
So please provide a mechanism to chage the trigger condition in the flow for a completly different trigger condition.
Today it is possible but you first have to remove the uses of the outputs of the trigger. At that point you can remove the trigger and add a new one. This is required because if you have actions that depdend on specific fields from the trigger outputs you need to find all of those locations and point them at the new trigger outputs fields once available.