If you use a gallery you can sum all the items in a column and display it in a text label using the follow code:
Sum(GalleryName.AllItems, ColumnName)
if you need to format the ouput to say dollars or something use the Text() function like so:
Text(Sum(GalleryName.AllItems, ColumnName), "$#0.00") this will show 58 cents as $0.58 etc.
Im not sure if you are using a form. If you are using a form then you would need to change the update property of each data card to use the combobox you added so that it knows what to use when it patches values to your SP List.
If you are not using a form then I assume you are using a patch statment. The only issue you may come across with that is if you are using complex column types in SP Lists such as choice and people columns etc. If you are not then you can just use a patch statment in the same way as normal. If you are using complex columns then you will need different syntax as you are upload an object as a value and not just a string or number. This requires some more code and structuring to achieve. Here is an example of a choice column patch statement:
Patch(TravelExpenses, LookUp(TravelExpenses, ID=selectedItem.ID), { StatusChoice: {Value: "Approved"} } )