How to update the First Business Object Field from a Second Business Object

Summary:

We have a Requirement where we are creating a new record in BO1 which has a Field named Flag which is posed as Y in BO1 along with a JOB_NO field.

Now in BO2 we are selecting the this JOB_NO field from a dropdown in BO2 and adding other fields and posting it to BO2.

In the BO2 save event we are trying to update the BO1 Flag =N using Get and Patch rest call of the BO1 but it is not updating the Flag =N for the BO1. BO2 Records is getting Posted

please advise any sample app