Maple has a number of built-in routines for evaluating iterated integrals numerically. The study of such methods properly belongs in a Numerical Analysis course, and will not be considered here. In fact, numeric methods for iterated integrals do not appear in the typical calculus text.
Just as for the numeric evaluation of single integrals, Maple applies the evalf command to the inert form of the iterated integral to effect a numeric calculation. This can be done either by command or interactively, via the Context Panel. The examples below illustrate the technique.
The Maple help page for numeric evaluation of definite integrals is available here.