I think you can change the icon by dragging and dropping
the icns file on the icon that appears in top left corner in the window
Get Info . For example, if I right-click
test.app displayed in the Finder on my Mac and select
Get Info I get the following pop-up window.
Below is a folder with some
os_centos.icns icon and drop the icon displayed in top left corner by
Get Info will produce the following change.
You can convert files to
.icns by visiting the site https://iconverticons.com/online/. Online conversion is free.