Find the UUID of the Channel Tag that you want to modify.
http://[TVH_IP]:9981/api/channeltag/grid
You can even fine-tune the query to only return Channel Tags that have a sort index of zero.
http://[TVH_IP]:9981/api/channeltag/grid?filter=[{"field":"index","type":"numeric","value":"0"}]
Update the Channel Tag object with the index value that you want.
http://[TVH_IP]:9981/api/idnode/save?node={"uuid":"[INSERT_UUID_HERE]", "index":"[INSERT_SORT_INDEX_HERE]"}