Log into your Control Panel and then click on File Manager. You will now see a list of directories within the root of your account. Since all of your html files and subdirectories are uploaded and created within your public_html directory, you need to click on the file folder next to the public-html directory name.
The directory will open and in the upper right hand corner, there will be a list of actions that you can perform for that directory. Next click on the file icon located next to the text name of the file that you want to change permissions for. Again, in the upper right hand corner you will see a list of actions that you can perform with this file. Simply click on Change Permissions, select the appropriate permissions and save.