I'm using a free app Pictogram (https://pictogramapp.com https://pictogramapp.com). It's not as feature rich as Replacicon (e.g. no online search from the app),

I'm using a free app Pictogram (https://pictogramapp.com https://pictogramapp.com). It's not as feature rich as Replacicon (e.g. no online search from the app), but it works well if you just need to change an icon or two. Services like https://macosicons.com/ https://macosicons.com/ help you to find a good icons online.