There are many different "Desire" models from HTC, so to give specific advice we'd need to know which. "Desire" with no qualifier would imply the original, 4-year old, 3.7" screen model, but I'd not want to assume that, especially since it must be 3 years since a software update for that phone was released but it sounds like yours was more recent.
If you want to completely reload your phone's software you want to use a ROM Update Utility (RUU). This is a Windows app which will check your phone's current software, and the original branding of the phone, and if these match (i.e. correct "customer id" and the existing software is not newer than the RUU) it will completely reload the phone's software. To run it, connect the phone via usb and run the RUU .exe on a PC. If in doubt put the phone in "fastboot" mode first (boot while pressing the "volume down" key, and when you get the white bootloader screen select "fastboot" from the menu).
Note: an RUU will wipe the phone completely (i.e. does a factory reset), so back up all of your important data before trying to use it.
You can find RUUs at
shipped-ROMs or
androidruu. Both of these organise their RUUs by device, but use the device codename in most cases (so e.g. the original Desire is "Bravo" in this catalogue). So you do need to know what you are looking for.
If you want more guidance, you could register and post some more information about your phone. Then we might be able to help identify the correct RUU for your device.
, i want to know if there is anyway to redownload a software update for an android phone ...or how to remove the whole software from the device and put it again fresh and new. 
arty: