how to subtract a negative fraction for a positive mixed number?????
Your meaning is not clear. It probably would have helped if you tried to give an example.
$$\\2\frac{5}{8}\;-\;- \frac{2}{8}\\\\ 2\frac{5}{8}\;+\; \frac{2}{8}\\\\ 2\frac{5-2}{8}\\\\ 2\frac{3}{8}\\\\$$