I have the following ring R of matrices

(a11 a12 a13

0 a22 a23

0 0 a33)

such that aij are in Z2.

R has 64 elements. Let G be the group of units of R. In this case, it consists of the matrices such that a11=a22=a33=1. Therefore G has 8 elements. Prove that G is isomorphic with D4.

How would I show this proof?