Follow the steps to install your WordPress theme
Install a Theme using WordPress Admin Theme Search
Check WordPress.org themes directory for free WordPress themes.
1. Log-in to your WordPress admin area. Next, click on Appearance, select Theme.
2.Click on the Add New button at the top.
On the next screen, you will have the ability to select from: Latest WordPress themes, search for a specific theme, search for themes with specific features or Featured WordPress themes, Popular WordPress themes,
Based on your search, you will see a single theme or a list of themes that met your criteria. In my case, I searched for the blogging theme Slipstream.
When you see the theme that you want to install, simply navigate to the top of the image. This will reveal the install button, preview button, and details button. Go ahead and click on the install.
WordPress will now install your theme and show you a success message along with the link to activate or live preview.
Click on the activate button, and you’ve successfully installed and activated your WordPress theme.
Note: Depending on the theme, there may be additional settings options that you may need to configure either through the theme customizer or through a separate options panel.
Install a Theme by using Upload Method from WordPress Admin
The first method above, only allows you to install free themes that are available in the WordPress.org theme’s directory. But if you want to install a “premium” WordPress theme, a paid theme, from companies like ThemeLab, StudioPress, iThemes, etc.
Or you want to install a custom theme then use upload method from your WordPress admin.
Start by downloading the .zip file of the theme that you purchased from the commercial theme provider.
Next log-in to your WordPress admin area and click on Appearance, select Themes.
Click on the Add New
On the next screen, click on the Upload Theme button at the top.
You will be prompted to choose the zip file that you downloaded earlier. Select the file and click Install Now.
Once your theme is installed, you will see a message along with the link to activate and preview the theme.
Click on the activate button, and you’ve successfully installed and activated your WordPress theme. Depending on the theme, it may add additional settings options that you may need to configure either through the theme customizer or through a separate options panel.
Note: The theme upload functionality is only available for self-hosted WordPress.org users. If you’re using WordPress.com, then you will not see this option because it limits you. In order to use custom theme upload feature, you would need to use self-hosted WordPress.org
Installing a WordPress Theme using FTP]
Connect to your host using FTP. You would need to go to the path (/wp-content/themes/). Once you’re there, simply upload your theme’s folder there.
Remember you must unzip the folders before you upload a theme using FTP. Once you have uploaded the theme, you would need to go to your admin area and click on Appearance » Themes.
You should see the theme that you uploaded listed there. Bring your mouse on top of that theme and click on the activate button.
You can also check How To Install A WordPress Plugin.