As some of the other posters have said, converting from NTFS to FAT32 is a Bad Idea if you plan to still boot into Windows. It's not supported by Microsoft, and for good reason -- there are a lot of significant differences between the two formats.
If you really, really need to convert, then back up the data off of your NTFS partition, delete and recreate the partition using FAT32 and restore your data from there. I still wouldn't do this because you never know how Windows will respond to having a sudden and drastic change like that.
Note that the kernel does have support for writing to NTFS partitions, though it's currently marked as DANGEROUS, which usually means stay away. However, thought I'd mention it as an option. (albeit a scary option.

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