You can install the theme as following 2 ways.
1. Download and Upload Manually
– Goto WordPress.org and themes library
– Search a theme writing the themename on search box.
– Download the theme (theme will be download as a zip file)
– Goto yoursite.com/wp-admin dashboard
– Then Appearance-> Themes
– Click on “Add New” then “Upload Theme”
– Choose themename.zip file (that you already downloaded in your computer) and click on “Install Now”
– Wait for a moment while processing and click on “Activate”
2. Install from WordPress theme library Directly:
– Goto yoursite.com/wp-admin dashboard
– Goto Appereance > Themes
– Click on “Add New” button
– Goto search field and type “themename” and press Enter.
– Now, hover over the theme thumbnail, then click on “Install” and wait for a while.
– Then click on “Activate” button.