Dealing with theme from WordPress themes repository
After logging in on the left sidebar click on Appearance by default themes your installed themes are shown. Click Add New in the top left of the screen.
Click on your selected theme for more information.
Click < to go to your previous theme or > to go to the next theme. This is great to see the theme in Live Preview, meaning you can see a predicated look at the theme. (Better to Activate the theme and check your website. You can always change back later). After you select you theme click Install. Next screen will ask you to Activate the theme.
How to Upload a theme
If you have purchased a theme or downloaded a free one. Go to Appearance >> Themes >> Add New >> then Click Upload Theme.
Once the theme is activated and you are happy.
Delete all unused or unwanted theme
To delete a theme. Go to Appearance, click themes and again you will see all you installed themes. Click on the ones you don’t want. Notice Delete in the bottom right of the popup window.
I always delete unwanted themes and plugins. Keep your website size down. Lean and mean as they say in Kerry.