When you make a partial with stacks, depending on the content you want to change, there is a small “pushpin” icon on the top right corner of the content area. If you select it that area becomes editable on each page you add the partial on:
I don’t know what PayPal button you are using, and what you need to change on the button. As a general rule, if you can edit the item in the main edit area and not the inspector sidebar, it’s pinnable or unpinnable. If what you want to change is in the inspector area then you will need to replace the entire item.
Most two-column stacks the entire content area for each side is pinnable.
Play around with it and you’ll see what I mean.
Hears a video you can have a look at, it’s on stacks 3 but still applies to Stacks 4: