upgrading version 10 to 12 told to uninstall 10 first then install 12 but will I loose all files if I uninstall 10 first?
There’s a whole other area of VMTN for Fusion, this is the area for NSX - thread reported asking moderators to move it,
No, your VMs will remain as they are.
Think of it like Word and your Word Documents. Deleting Word doesn't delete your Documents, and Virtual Machines are basically fancy Documents.