Steps to change the category of a product:
Go to 'Products' and click 'Edit' for the product whose category you want to change.
Go to the 'Category' section.
Click the 'x' to remove a category and click the blank field to assign a new category to the product.
Then Click on 'Update Product'
Steps to add a new category to a product:
To add a New category, simply type your desired category in the blank field and click the plus icon. You can then select this category.
You can also add a new category using the 'Manage Categories' button which is present in the Product Screen. This would give an overview of all the categories and you can delete a category as well from here.
⚠️NOTE: You cannot edit the Category of the product once a PIM tab is created.