https://docs.atlas.mongodb.com/setup-cluster-security/#add-to-route-table

*_Add the Atlas VPC’s CIDR block to the peer VPC route tables.

Once set up, you can edit or terminate VPC peering connection from the Peering table._*

This is a statement and really does not provide a guide on how to do so. A more effective method would be to use use the information in section 12 and 13 in some form from https://docs.atlas.mongodb.com/setup-cluster-security/#add-to-route-table

If you have more questions on this, feel free to contact me.