If you need to quickly re-sync a given product or collection in Shopify, you can use the 'Sync with Algolia' action that is available on the Shopify admin for your products and collections:
1 - Re-sync a product:
Product > More actions > Sync with algolia
2 - Re-sync products using the bulk editor from Shopify:
Products > More actions > Sync with algolia
3 - Re-sync a collection:
Collection > More actions > Sync with algolia
4 - Full reindexing
Finally, if you need to re-sync your whole data, you can trigger a full products reindexing.