1. If you are using ServiceDesk Plus-MSP 8.1 version, then first you need to upgrade to 8.2
Please use the below link to download the servicepack and follow the usual upgarde procedure,
2. Take a backup of the application.
3. Make a fresh install of 64 bit version.
4. And now restore the backup data.
Note: Please make sure you have made a copy of the backup before uninstalling the application.