Shopify admin action “ALTSEO – Update Alt Text” fills missing alt only — it does not overwrite existing text.
From a product in Shopify admin you can run ALTSEO – Update Alt Text. It only targets images missing alt text and never overwrites.
If you see “No images require alt text,” every image already has alt. Use a bulk New Task with overwrite if you need a rewrite.
Related help article: Read the guide →