To update a metafield, you can use the “Update a Metafield” module, twice (once each for meta title and meta description)
You should have something similar to this:
I can’t actually test it out because I don’t have access to your Shopify store. For more assistance on using the correct Metafield Namespace and Key to update the values (meta title and meta description) for your Shopify store’s blog posts, please contact Shopify support.