Certified GKH Gardening Expert
You can use things such as copper antifungal solutions, or you can use wettable sulfur. Either of these will take care of the issue, as long as it isn't one of the "incurable" species of fungus. I prefer wettable sulfur, as it provides things that the tree will likely need anyways.
You might want to add dolomitic lime as well to keep the pH in the soil a little more stable while this is working on the soil.
Here is an article that will help: https://www.gardeningknowhow.com/edible/fruits/apples/apple-collar-rot.htm