Credits to @andreyaceleste
Go into superimpose studio
Press "layer"

Ignore the photos I have a project on Ancient Rome...
We had to do a info graphic ;)

Click on the layer, then cut

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Auto

If the background is a solid color, then using the auto tool can get rid of the background
Try the auto first. If that doesn't work, press undo and check out the "lasso" section below.
Press done if finished

Lasso
Outline the subject with the lasso tool
Outline until you reach your starting point
Adjust the delete level to your preference (there will be no delete level if using auto)
Ex out of the delete level
Press done

Eraser
If that doesn't work, you can use auto on small parts and the eraser which just erases the part.

My Ancient Rome project if anyone wants to see that:
I cut the two signs and the colosseum at the bottom