To change a theme all you need to do is login to the wordpress admin area.
Click on the “Appearance” menu and then “Themes”.
There are two tabs there one is to manage the theme you have installed and the other option is to install a new theme.
Since you want to change your theme click on the “Install Themes” tab and you are good to go.
You have two options available to you there.
1. Select a theme from the WordPress theme directory.
2. Upload your own theme.
That is all you need to know to change the theme of your WordPress Site