It depends on what you want to edit or change. Basic styles/appearance can be changed by editing a themes .css file(s).
Modifications beyond CSS or HTML will require some knowledge of how php is used to retrieve and display content within the context of the WordPress framework. Creating new or modifying page templates is often a way to incorporate personalized changes into an existing theme. Most frequently, you will find that creating a child theme is the most effective - and safest - way to make modifications to an existing theme. It prevents the loss of your customized theme when an existing (parent) theme is upgraded.
