As this seems to be homework I am just going to help you do that. I am not going to solve it for you.
You need to write that as a Riemann sum for some function. Look at the general term of your sum. After multiplying 1/n through we get it in the following form:
(1/n) * (i/n)^3
What is a Riemann sum of a function f(x)? It is the sum of terms in form f(xi*) * (xi – x(i-1)) Where xi* is a sample point in the i-th interval [ x(i-1), xi ].
Here is a hint for your example: (xi – x(i-1)) in your example is 1/n. Figure out the function f(x) that its Riemann sum would give what you are looking for, by looking at (i/n)^3.
What interval has been divided into n small subintervals of equal length? That would give you the lower and upper limits of your definite integral.
Let me know if you need more help. Good luck!