When you cannot use wp-admin, or you need to upload plugin files by hand, use your host’s file manager or FTP/SFTP.
A host file manager in the control panel is usually the fastest option. Not every host offers one.
FTP (or SFTP) lets you upload, download, rename, edit, or delete files on the server. You need an FTP client and the host credentials (server, username, password, and often a port).
WordPress documents the process here: Using FTP Clients.
Be careful. Deleting or moving the wrong file can take the site down. Prefer SFTP when your host supports it, and keep a backup before large changes.
