In order to install an extension for Magento 2, normarly you have to use the command line through SSH. Howver you can install it without SSH.
For example: install PL_Payment extension
Step 1: Open Config.php file in app/etc/ then add a line code
Step 2: Add record into database, the table named setup_module .
Access directly to database by phpMyAdmin or something like that.
Step 3: Now you need to refresh or flush cache.
Done!