Download and Upload your preferred the Theme
Once you have downloaded the Magento 2 theme of your choice( purchased or free), you will receive a zipped version. Unzip it and upload the files to the public root directory of your Magento 2 store.
Note: You can use FileZilla for it too.
Connect Store with SSH
Now go to the root directory of your store via SSH.
Upgrade the setup by running this command:
php bin/magento setup:upgrade
Now deploy the static content by running this command:
php bin/magento setup:static-content:deploy
Activate the Theme
Navigate to CONTENT → Configuration from the Admin Panel of your store.
Click on Edit to apply the theme.
Here, you can change and activate your theme.
Select your theme from Applied Theme option and then just click on Save Configuration.
Go to your web browser and run your store, and you will get your newly applied theme.