Tap the Tune slider icon at the top right of your For You feed to open your Personalize options. From here you can add and remove topics. It will suggest related topics you might want to follow as well. The more specific you make your topic selections (example: following #Italian Cooking vs just #Food), the more fine-tuned your feed can be.
If you choose to remove a topic in For You, you will unfollow it everywhere. This includes that if it was pinned to your top home bar it will be removed.
In addition to this Personalize screen, you can add topics and magazines in the Explore search tab and remove them in the Following tab. Remember that For You will pull from all the topics, magazines, and other sources you follow. The more thoughtfully and specifically you refine what you follow, the more targeted For You will become.