First, write the meaning, the definition, of f IS an increasing function. Then negate it, since it's NOT true that f is an increasing function.

Meaning of "f IS increasing" is given by "For all x1, x2 in R, with x1 < x2, have _______" (fill in some condition C(x1, x2, f)).

Meaning of NOT "f IS increasing" is therefore "There exists x1, x2 in R, with x1 < x2, such that _______" (now fill in the condition NOT C(x1, x2, f)).