To edit an attribute in your collection, go to the Attributes tab in your collection's details window and click edit to the left of the attribute you want to edit.
The process of configuring attributes when they're created and re-configuring attributes when editing them is the same. However, when an attribute has been created, it can only be switched to other attribute types that are similar to it.
For example, an Image Upload field can be modified to become a file upload field, but it cannot be changed into a paragraph text field.
View the documentation on attribute types for more information.