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FTP is short for "File Transfer Protocol". A great and free FTP client is FileZilla. Download it and install it. Note: do not download FileZilla Server, only FileZilla Client!
Once FileZilla is installed, you will need to configure it with your site's log-in parameters, which should have been sent to you by your host. If not, ask for them to your host – or check your spam folder 
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- Press Ctrl-S.
- Click the "Open the Site Manager" icon, at the top left.
- Open the "File" menu, and select the "Site Manager..." option.
A window opens.
To add your hosting space to the Site Manager:
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In any case, thanks to FileZilla (and most FTP clients), you can change permissions easily and graphically: once you have found a file or folder that needs such a change, right-click on it from your FTP client, and in the context menu choose "File permissions...". It will open a small window.
Depending on your server configuration (which you don't always have a hand at), you'll need to both "Read" and "Execute" columns of boxes checked, and at least the "Owner" and "Group" rows for the "Write" column. Some host might require you to have the public "Write" box checked, but be careful with that: have anyone be able to edit the content of your folder is rarely a good thing.
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