I know you can create a new index through REST:
curl -k -u admin:changeme https://127.0.0.1:8089/servicesNS/admin/search/data/indexes -d name=mynewindex -d frozenTimePeriodInSecs=#######
Is there a way to change or update an existing indexes frozenTimePeriodInSecs though REST?
Let me know if the suggestion below worked.
This absolutely can be done.
Based upon your example here is what you would do for the case of your index named mynewindex wanted to be changed to a year of retention:
curl -k -u admin:changeme https://127.0.0.1:8089/servicesNS/admin/search/data/indexes/mynewindex -d frozenTimePeriodInSecs=31536000
Specifying it in the path will alter the settings on that index via the REST endpoint.