nextcloud/apps/files/appinfo/update.php

<?php

// fix webdav properties, remove namespace information between curly bracket (update from OC4 to OC5)
$installedVersion=OCP\Config::getAppValue('files', 'installed_version');
if (version_compare($installedVersion, '1.1.4', '<')) {
	$query = OC_DB::prepare( "SELECT `propertyname`, `propertypath`, `userid` FROM `*PREFIX*properties`" );
	$result = $query->execute();
	while( $row = $result->fetchRow()){
		$query = OC_DB::prepare( 'UPDATE `*PREFIX*properties` SET `propertyname` = ? WHERE `userid` = ? AND `propertypath` = ?' );
		$query->execute( array( preg_replace("/^{.*}/", "", $row["propertyname"]),$row["userid"], $row["propertypath"] ));
	}
}

//update from OC 3

//try to remove remaining files.
//Give a warning if not possible

$filesToRemove = array(
	'ajax',
	'appinfo',
	'css',
	'js',
	'l10n',
	'templates',
	'admin.php',
	'download.php',
	'index.php',
	'settings.php'
);

foreach($filesToRemove as $file) {
	$filepath = OC::$SERVERROOT . '/files/' . $file;
	if(!file_exists($filepath)) {
		continue;
	}
    $success = OCP\Files::rmdirr($filepath);
    if($success === false) {
		//probably not sufficient privileges, give up and give a message.
		OCP\Util::writeLog('files','Could not clean /files/ directory. Please remove everything except webdav.php from ' . OC::$SERVERROOT . '/files/', OCP\Util::ERROR);
		break;
    }
}