late to the party here!

I can't see your Zap setup, but the ability to delete multiple rows as outlined in the Google Sheets apps page here: https://zapier.com/apps/google-sheets/integrations

If that doesn't work for you, transfers might be the next best option. This does mean that you would need to manually delete all rows on your Google Sheet B though. This should be easy enough to select all cells in the spreadsheet and then delete them all in one go, then setup the transfer.

You can transfer existing data from Google Sheets. Here's a guide to help get started: https://help.zapier.com/hc/en-us/articles/8496274335885-Transfer-existing-data-using-a-Zap#h_01HJ8K1BGJ8X1D47RHVNM587JH

This guide might be helpful as well to get an overview of data transfers: https://zapier.com/blog/zapier-transfer-guide/