Define the tree, such that the parent node always contains the sum of children nodes.

Simple. I think, no explanation is required for this.

Get summation values of right sub-tree and left sub-tree, and add them to current node's data and update.

/*Define the tree, such that the parent node always contains the sum of children nodes.*/
int sumUp(struct node *root){
 if(root==NULL){
  return 0;
 }else{
  int l=sumUp(root->left);
  int r=sumUp(root->right);
  root->data=root->data + l + r;
  return root->data;
 }
}