The easiest way to chmod files (change permissions) is using the FTP client. Log on to your FTP account and select all of the files/directories you want to change permissions for. Right-click the selected files and select 'File Attributes' (in Filezilla) or 'Properties' (in most of other FTP clients) menu. You will see some numbers like 644 or 755 (for example) in the file attributes window. Change these numbers to any value you need (e.g.777) and click the OK button to confirm the changes.
You can also change file permissions by using the File Manager which is located on your cPanel. The button for permission change is located on the top toolbar in the File Manager window.