Certain file permissions are required for MyBB to function correctly. Once you’ve uploaded your files you will need to set the permissions on certain files and directories.
** On new installs (not necessarily for upgrades), before applying file permissions, rename config.default.php to config.php
*nix systems via CHMOD
If you have SSH access, you can apply the necessary permissions via the following command, executed from your root MyBB directory:
chmod 666 inc/config.php inc/settings.php
chmod 777 cache/ cache/themes/ uploads/ uploads/avatars/
Optionally, you can also apply the following permissions:
chmod 666 inc/languages/english/*.php inc/languages/english/admin/*.php
chmod 777 cache/ cache/themes/ uploads/ uploads/avatars/ admin/backups/
If you are using FileZilla you can right click on a file or directory and click File Attributes to modify the permissions of that file.