Hi @Josh1,
There is no direct action and Expression in Flow to change an element of an array currently.
If you want to function that you mentioned to be supported in Microsoft Flow, please submit an idea to Flow Ideas Forum:
https://powerusers.microsoft.com/t5/Flow-Ideas/idb-p/FlowIdeas
In addition, I use a series of variables to change a certain element, the process is more cumbersome, there may be a better way, but you could refer to this method first:
The general idea is to manually add an index to the array, then change the array element corresponding to the specified index, empty the previous array, and add the new element.
Array-Test: the initial array
Array-Index: create an index
Array-Tran: As a transition, store the Test array elements and Index, one-to-one correspondence.
Traverse the array-Tran and add the Value as a new element of the Test array.
If the index is equal to 3, need to add 1 to Value by using add() function.
Image reference:

Apply_to_each:

Apply_to_each 2:

Finally, you could output an array to see if the result is correct.
Please take a try.
Best Regards,
Barry