Mixtable's spreadsheet interface makes it trivial to bulk edit tags in your Shopify store. Here's how.
Set up a workbook and set up a worksheet to sync the data whose tags you'll be editing. In the examples below we are using a worksheet that syncs our store's Products, and so we'll be editing Product tags.
Right click on the column header of a column that is not yet mapped to sync to a field. Such columns have a grey background, whereas columns that are set up for data syncing have a light purple background. When you right click you'll see a context menu show up. Select the Edit Column Settings option.
In the window that opens, select the Core Field radio button option. You will see a dropdown show up, prompting you to select the field you'd like to sync. Find and select the Tags field. Now, click on the Save button.
You'll now see the column populate with any existing tags for your products.
Now, find the product whose tags you want to modify. You can set tags directly or use an Excel-style formula. If your product has multiple tags, you need to separate those with a comma (e.g., "tag1, tag2") as that is how Shopify stores tag information.
To replicate the tags of one product to others, simply copy and paste the relevant cell.