You will first need to make sure you have SSH access to the account you want to fix permissions on
Here is a guide on how to access the account on the server via SSH.
Files - 644
Directories - 755
The /public_html/ folder should always be 750. Be sure not to change this.
To fix permissions you will need to run the following command line:
chmod XXXX FILENAME/DIRECTORY
Replace XXXX with the permission number and FILENAME/FOLDER with the name of the file or folder you are changing permissions for.
Please keep in mind that it's best to backup your account before you change any permissions to prevent any issues from occurring.