Alter a 3D Model from STL file

Alter a 3D 'flip flop' design from the STL file. I am imagining a flip flop that has a regular ethernet connection port as the point where the flip flop straps attach to the sole of the flip flop. So, 1) Change the holes where the straps enter the sole (female) to a standard ethernet connector (ie, the port on a computer) 2) Change the flip flop strap so that they have an ethernet connector (male) attached to the end points (ie, the part that goes into the computer) 3) Have the flip flop strap as a separate piece to the flip flop sole 4) The depth of the flip flop sole must be deeper than the depth of the connection.