The term Micropenis is just a term to describe an unusually small penis in males.
For an adult if your penis is less than 3 inches long when fully erect then your penis would be considered abnormally small and be known as a micropenis.
There's nothing you can do to enlarge your micropenis unless you do surgery however that's not recommended.
However a Micropenis can restrict the abilities of a man to have normal intercourse but there's nothing much you can do about it.