Answer:
Replace 5 and 4 with 6s. Replace 2 and 3 with 1s. Then there will be 3 faces with 6 and 3 faces with 1.
Step-by-step explanation:
In order for the mean to remain unchanged, the sum of opposite faces must remain the same: 7. In order to have the standard deviation as large as possible, the largest and smallest possible numbers need to be used: 6 and 1.
Replacing 4 and 5 with 6s, and replacing 2 and 3 with 1s will accomplish your goal.