nextcloud/apps/calendar/appinfo/update.php

<?php

$installedVersion=OCP\Config::getAppValue('calendar', 'installed_version');
if (version_compare($installedVersion, '0.2.1', '<')) {
	$stmt = OCP\DB::prepare( 'SELECT `id`, `calendarcolor` FROM `*PREFIX*calendar_calendars` WHERE `calendarcolor` IS NOT NULL' );
	$result = $stmt->execute();
	while( $row = $result->fetchRow()) {
		$id = $row['id'];
		$color = $row['calendarcolor'];
		if ($color[0] == '#' || strlen($color) < 6) {
			continue;
		}
		$color = '#' .$color;
		$stmt = OCP\DB::prepare( 'UPDATE `*PREFIX*calendar_calendars` SET `calendarcolor`=? WHERE `id`=?' );
		$r = $stmt->execute(array($color,$id));
	}
}
if (version_compare($installedVersion, '0.5', '<')) {
	$calendars = OC_Calendar_Calendar::allCalendars(OCP\USER::getUser());
	foreach($calendars as $calendar){
		OC_Calendar_Repeat::cleanCalendar($calendar['id']);
		OC_Calendar_Repeat::generateCalendar($calendar['id']);
	}
}