Tefillah doesn’t change Hashem’s mind, rather,

“When one prays, he rises to a higher place, and becomes a new person. And although Hashem decreed on the previous person a certain decree, with tefillah to Hashem…he became a new essence.

It isn’t that Hashem changed his desire, rather on this person who is presently here, the decree was never placed.”

~ The Bnei Yissaschar zt”l