How can a beginner remove an actor from footage?
All of these seem to be simple circle mattes, you just need an editing system that allows you to generate a circle mask that has an animatable position. 

You will need 3 pieces of media:
1- the original footage
2- the new actor to go in the circle (or a neutral shot)
3- the moving circle mask

You will need to animate the circle to follow the original circle, then use the circle mask to reveal the new footage in the hole.

I can recommend using the Open Software "Blender", it has a Video editor called the VSE and a system of user defined masks that you can animate with.