A bit more detail would be helpful, but you will need a common column between the two lists. Then, when a change is made to the first list, you use a Get items action using an OData filter to retrieve the item in the second list - that will give you the list item ID. Then, use an update item action to update the second list. You'll also use an expression when updating to come up with the updated value.
I can provide an example if you provide more detail.