Another topic-how can I export an objects obj from sweethome programme to back up file/external harddrive?

Select the object and choose the 3D view > Export to OBJ format menu item. Then, when prompted select Export selection option.

If I was updating the sweethome software for a new version-would those previously downloaded object stay in my sweethome software?

Yes, imported objects and textures are saved in private folders that are not deleted when you uninstall Sweet Home 3D or install a new version.