Essentially, your options boil down to doing one of two things: reformatting the drive or partition (which results in the data being destroyed) or shelling out for a third-party program. Unfortunately, Windows does not have a built-in tool to convert backwards, like one can to convert from FAT/FAT32 to NTFS. Enter Partition Magic. Partition Magic allows you to convert an NTFS partition back to FAT32, but there is a reason that Partition Magic is not so lovingly referred to as "Partition Tragic". Often conversions simply fail, or worse, destroy data.
This guide is to walk you through the former method of conversion: deleting a partition, and recreating it anew as FAT32. However, no matter which route you choose, backing up your data should be your first priority.
First off, delete the partition that you wish to convert. This is done by going into Disk Management: right click "My Computer" -> Manage -> Disk Management, which is found under the Storage section. Right click the partition you wish to remove, and click "Delete Volume". This will erase the partition. Once you have done this, you must re-create the partition. This is done by right clicking on an unallocated region of a disk, and selecting "Create Partition". Then click "Create Logical Drive". Bear in mind that Windows cannot format a FAT32 partition that is any larger than 32GB. This is the case because FAT32 is terribly inefficient on volumes that are larger than 32GB: fragmentation becomes a serious problem.
To format this new volume, right click it, and choose Format. Again, if the volume you wish to format is larger than 32GB, FAT32 will not be one of the options available to you in the drop-down box. You will have to create multiple partitions if you want to format a large drive as FAT32.