Hi and welcome to the forum
You can use the command prompt to do this. The command is convert, so you should type something like this:
convert K: /FS:FAT32
K: is only an example, you have to use the letter assigned to the external HDD once you connect it to your computer. I can't remember off hand if you have to type K: or K only (without the ":"). Try both and see which one is the right one
Bear in mind, however, that converting a hard disk partition or volume from NTFS to FAT32 requires that you reformat the partition, which deletes any data on it. Be sure to back up any data you want to keep before you begin.
Moreover, windows doesn't allow to create FAT32 partitions larger than 32 GB. So if you want to format the entire disk to FAT32, the only reliable way I can suggest at the moment is to use Linux to format the disk. Linux doesn't have such limitation.
If you don't have a computer running linux, you can use a Live CD like Knoppix or Ubuntu. Simply, boot your computer from the live CD, then connect the external drive to the PC, and format it to FAT32.
Don't worry: Live CDs will not change in any way your computer, because everything is done from the RAM and from the CD