How to apply as the corollary showen in the following example

since 1 is generator of this group. here a = 1, now check powers of a which are relatively prime to n=6 That is only a^5 = 5 only two generators.

Consider those elements in the group, that are relatively prime to 6. 1 and 5, satisfy the condition. You can verify whether they generate all the elements of the group.