It would be helpful to know what kind of update you are wanting to make to the Parent. If your Child changes are being made in the app from a form, there is a good chance you can update the parent there as well.
Since you have implied that you want the Parent change AFTER the child row is Saved then I’ll continue with that assumption.
I’ll assume you have the “IsPartOf” flag turned on for the Parent/Child relationship.
To make changes to the parent fields when a Child row changes you’ll need to create a Workflow that then calls an Action, call it Action 1.
In Action1 make it of type “execute an Action on a set of rows” and transitions to the Parent row so you can access the parent fields to make updates. The Reference Rows expression is simply [Parent ID].
Create a second Action, Action2, of type “set the values of some columns in this row” and assign it as the Reference Action in Action1.
Update Action2 to select the columns you want to update.
If you are wanting to update with information from the child row that just changed, you will need someway to identify that row so you can implement a SELECT() statement in the parent fields being updated to choose the value from that child.
I hope I have made it clear and it helps.