For a single site WordPress installation use the Dashboard > Plugins page.
- Find plugins using WordPress.org.
- Download .zip file to your local machine
- Use Add new > Upload
- Search from Dashboard > Plugins.
- Choose Install
In WordPress Multisite use My Sites > Network Admin > Dashboard > Plugins Dashboard > Plugins to manage your plugins.
Before doing anything take a backup!
If things go badly wrong see Recover from a WordPress Fatal error using your website’s control panel (cpanel)