Here is the footnote from the Aquinas Study Bible
3:9 God may yet repent: Divine repentance takes in all cases a different form from that of man, in that it is never regarded as the result of improvidence or of fickleness, or of any condemnation of a good or an evil work. For it will have no other meaning than a simple change of a prior purpose; and this is admissible without any blame even in a man, much more in God, whose every purpose is faultless. Now in Greek the word for repentance METANOIA is formed, not from the confession of sin, but from a change of mind, which in God we have shown to be regulated by the occurrence of varying circumstances. (Tertullian) God is said ‘to change His mind,’ metaphorically, inasmuch as He bears Himself after the manner of one who repents, by ‘changing His sentence, although He does not change His plan. (St. Thomas Aquinas)