As you mentioned that you have already downloaded the application, you can launch it and select the NTFS partition you want to convert. Click Convert to initiate the process.
It is recommended that you connect the hard drive. Open Computer from start menu or the icon present on the desktop. Right click on the drive you want to change the partition for. Select Format > choose the File System you want for the hard drive. In your case, it would be Fat32. Format the drive and the file system will change.
NOTE: Formatting the drive will remove the data stored in it.
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