Using ftp to upgrade wordpress plugins is not a safe way: you will need to open the ftp port and have a ftpd daemon just for upgrading wordpress.
So instead we use ssh to help us upgrade wordpress plugins, first you will have php with ssh2 supported:

Next, login in the wordpress user account (the owner of your wordpress directory) and create ssh public/private keys:

Alternatively, if you want to keep the user more secure (i.e. using different public/private keys in different directory say wordpress-ssh/):

Done! When we are asked in the ‘Connection Information’ page, just select SSH2 and input the user and the public/private key path. If you want to have wordpress to fill it automatically, you can specify the following information in wp-config.php: