Please note that this Support article contains frontend customizations via modifying the legacy Algolia template files, which are deprecated and unsupported by the Algolia Support Team.
For more information, please review this Support article - "Why is modifying Algolia template files for customizations not supported?"
To change the index that is being queried in the Autocomplete dropdown in Shopify, you'll need to edit the algolia_autocomplete.js.liquid file.
Go to Themes > Current Theme > Actions > Edit Code.
Under Assets we select algolia_autocomplete.js.liquid:
On line 57 we find the "index" function:
An example change is below:
We then press "Save" to execute the change.
We do not recommend making this change without first discussing the impact with the Algolia Support team.