How should I cite it?
Do I have to cite it?
Should I cite it like "from [source] (modified)"?
You should indicate that the figure was taken from an existing article and that you have modified the figure. My usual means of doing so includes
Figure adapted from Fig X. in Ref. YY.
inserted at the end of the caption.
You should also make sure to include whatever copyright notice the journal or repository in question expects to be added.