In order to upload files to your Web Hosting account, you should:
1) Log in to your cPanel
2) Click on the "File Manager" icon
3) Visit the folder that contains your website. This is usually public_html if this is for your primary domain.
4) Click the "Upload" button to upload files.
If you are uploading a Wordpress plugin, you should upload it to the /wp-content/plugins folder. If you are uploading a Wordpress theme, you should upload it to the /wp-content/themes folder.
Please remember that if you upload a .zip file, and you extract it, you should delete the .zip afterwards to prevent leaking sensitive data.