You will first need to install Family Tree Builder on your computer. You can download it here.
If you have previously synced your family tree online, then there will be a backup of your original family tree that you created in Family Tree Builder and synced to your family site on MyHeritage. In that case, please follow these steps to recover the file:
Open the Family Tree Builder and go to File > Restore project > Restore from online family site > select a tree > Download now.