nextcloud/apps/files_versions/ajax/rollbackVersion.php

<?php

OCP\JSON::checkAppEnabled('files_versions');

require_once('apps/files_versions/versions.php');

$userDirectory = "/".OCP\USER::getUser()."/files";

$file = $_GET['file'];
$revision=(int)$_GET['revision'];

if( OCA_Versions\Storage::isversioned( $file ) ) {
	if(OCA_Versions\Storage::rollback( $file, $revision )){
		OCP\JSON::success(array("data" => array( "revision" => $revision, "file" => $file )));
	}else{
		OCP\JSON::error(array("data" => array( "message" => "Could not revert:" . $file )));
	}
}