In real life, there are no square signals.
Everything has a rise time, a slope. Lots of things ring and bounce.
For understanding the digital signal, you just pretend they are square!
That is, unless the signals are showing problems, then you pay attention to the analogue reality.
In your particular case, perhaps your scope probes aren't tuned, and you are introducing the ringing with the probes. Many oscilloscopes have a test signal and you can adjust the probes. Digital oscilloscopes have many other settings such as various interpolations -- for which see pipe's answer.
The non-squareness of your signal doesn't look particularly awful, but as commenters note, the ringing in advance of change is probably an artifact of the scope, not real in the signal.