No, you can only use it for what it states. If you use it for fewer nights you will forfeit the remaining nights. You can call Marriott, pay additional points and have the cert changed to a different category. So if you want a Cat 6 hotel call Marriott to change the cert.

If you presented a Cat 5 cert to a Cat 6 hotel, the property would not be able to get payment from Marriott. The cert is not points and a Cat 6 hotel would not be reimbursed for something it should not accept and may be sanctioned by Marriott on top of that.