Here are all the creations you've saved/downloaded.
This is where your info's stored (so, ya know, be careful!). Go in there, then 📂Scrap Mechanic, 📂User, and finally 📂User_xxxxxxx (where xxxxxxx is some random-looking number).
You can leave Scrap Mechanic open while you do the rest of this.
Anything works, as long as it's saved on your machine.