Don't let the OS to think it is smarter than you.

However, when it comes to computers, I am a complete idiot!

So do I!
Even so, I'm the master(as stupid I am) and the bloody computer is the slave(as smart as it think it is). That's my rule.
1. Use Synaptic (or whatever package manager you use) and remove(purge) all files related to SH3D provides by software app.
2. Download and extract it somewhere in your home user space.
You will have a folder SweetHome3D-7.3.
3. In that folder double click on SweetHome3D file and choose Execute.
That's the start.
If the OS still think is smarter than you, ignore it. It is not!.