You can do this with any FTP program, such as CuteFTP, SmartFTP, or FileZilla (the last one is completely free). In most FTP programs, changing file permissions is as simple as right-clicking on the file, and choosing Permissions or CHMOD from the menu.

Note: You should not be using "world-writable" (777 or 666) permissions or any other values that allow writing from everyone as this poses a security risk for your files and account.