You should login to your server via ssh, switch to user root and modify this file:
/usr/local/hestia/web/fm/configuration.php
Replacing this:
$dist_config["frontend_config"]["upload_max_size"] = 1024 * 1024 * 1024;
By this (it will raise the file size limit to 2GB):
$dist_config["frontend_config"]["upload_max_size"] = 2 * 1024 * 1024 * 1024;