If by changing the index you want to change it to go to default main index you can definitely follow these steps mentioned in readme
By default, the IMAP app will create a new Splunk index named "mail". This is controlled by: default/indexes.conf. If you want the IMAP output to go to the default Splunk index:
1. Remove "index = mail" reference in default/inputs.conf
2. Delete the default/index.conf file.
3. Comment out the "definition = index=mail" in default/macros.conf file.
4. Restart the Splunk server.
For any other index except default main index, it might be worth trying below without any guarantee that it will work or not and instructions below are pure assumptions and not recommendations (so try at your own risk 🙂😞
Try replacing references to mail index with yourNewIndex in the above two config files (and any other conf files where you see a reference of mail index).
Thereafter you can ensure that corresponding directory of index is present under $SPLUNK_HOME\var\lib\splunk\yourNewIndex and restart Splunk server to see if it works out or not.