Just traverse the node from root to left recursively until left is NULL. The node whose left is NULL is the node with minimum value.
Read full article from Find the node with minimum value in a Binary Search Tree | GeeksforGeeks
int minValue(struct node* node) { struct node* current = node; /* loop down to find the leftmost leaf */ while (current->left != NULL) { current = current->left; } return(current->data);}