How to update parent table identity column values in child table

Hello,

I have parent and child tables and already populated with data. We have identity column in parent table and we are using this column values in child table. 

We found an issue that parent table identity values are not sequence because we deleted some records and insert another records without reseeding the identity.

Now we have reinserted all the data into parent table and identity values are now in sequence. But we have child table with huge records with old parent id values and we need to update those values with new one.

Can someone provide any solution on this?

Anil