STL editing.

[Tri-Polar]

Hi gents,

I apologize if this is the wrong place for this and I specify joined to ask this as well.
I have been printing other peoples designs now for some time. I'm only new and trying to learn as i go.
I have been searching google and youtube to try and point me in the direction of how to edit a STL file.
Sure i could ask someone to do it, but i want to attempt to learn to do it myself. The thing i want to do is just fill in a indentation so the face is smooth. This may be a simple task or very hard, i'm not sure.
I have attached a photo of what i am trying to fill in.
I currently have Blender, Meshmixer, Meshlab and Openscad installed, but happy to try other programs.
Is there by chance anyone can explain or point me in a direction to do this task?

Thanks in advance.