Plugin options are normally stored in the wp_options table. If you look at the plugin code (and reference the Plugin information in Codex; you can start here: https://codex.wordpress.org/Writing_a_Plugin ), you will see how the plugin names the option. Then you can create a function that will change that option value.
Note that most plugins store their options in one row of wp_options table, so you will need to make sure you don’t bork the other options that the plugin uses.
You might also want to contact the plugin’s support area, which is the proper place for questions about plugins.