For most Key Results types there are actions available from the menu in the Key Result panel to allow you to change them.
Milestones Key Results cannot be changed, so you have to delete and add it again if you want to change the Milestones
Change Target
You to change the change Change, Target & Start Value and the Calculation used
Change KPI
For KPI based Key Results this allows you to change the KPI that the Key Result uses and then configure it.
To change the Key Result type see How to change the type of Key Results