Plugins
Step 1: Log in to your WordPress Dashboard. On the left panel, look for Plugins.
Step 2: Under the Plugin you wish to remove, you will see the word 'Deactivate'. Click on Deactivate and click Delete.
Themes
Step 1: Log in to your WordPress Dashboard. On the left panel, look for Appearance > Themes.
Step 2: Navigate to the Theme you wish to remove and click on Theme Details. At the bottom right hand corner there is a Delete button.