Change records in DNS slave if master is offline

[Roberto Carna]

Dear all, I have one BIND9 server as master and 3 as slaves.

The master and one slave are in a given site #1, and the other two
slaves are in a geographical different site #2.

In case site #1 goes offline, I need to edit records in both slaves
from site #2, in order to point some services to other public IP's for
contingency.

My question is:

What is the recommended way to edit the records from a BIND9 slave?
Because the zone files are binary files and using the Webmin interface
is blocked.

The only manner is changing the configuration from slave to master?

Thanks in advance, greetings!!!