Please refer here for how to change permissions, and which folders should be set to which permissions:
It is never a good idea to set a folder/file to anything more permissive than 755, especially if it's public facing.
If you have shell access use the chmod command, if you're using SFTP/FTP then you can set the permissions using your FTP client, refer to the manual or help documentation of your FTP client for instructions. Most programs will have this option when right clicking on a remote folder/file.
If you are unable to modify the permissions, contact your administrator/support, or if you're in a shell, try sudo