If you need to create or modify an .htaccess file, you can do it by modifying the file locally and uploading it by FTP. Also, you can use the Control Panel's File Manager to create, modify and delete files.
If you uploaded a .htaccess file that contains some incorrect instructions, you will get a 500 server error. You can find out what is wrong with the file by looking at the apache error log located in statistics/logs/error_log
