Do not worry if you have clicked left and created a straight line instead of a curve or vice versa.
Instead of doing it all over again, you can press the "Backspace" key on your keyboard to delete your latest point.
If you do want to start again, you can press the "Escape" key on your keyboard and delete the process of the object you were working on.
These two options only work if you have not yet pressed the "Enter" key
to set the sitch.
There are also several options on how to reshape objects after pressing "Enter"
Watch this tutorial
if you wish to learn more about reshaping an existing object.