Mixtable's spreadsheet interface makes it trivial to modify existing metafield values in your Shopify store, and to create new one. You can do this individually, or you can edit your metafields in bulk. Here's how.
Set up a workbook and set up a worksheet to sync the data whose metafields 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 metafields.
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 Shopify Sync Settings option.
In the window that opens, select the Metafield radio button option
You'll see that you are now being prompted for information on the metafield that we'll map into this column Each metafield in Shopify has a Namespace and a Name. Enter the values for any existing metafields here. If you are creating a new metafields then enter the Namespace and Name values you'd like to use. Then, click the Save button.
You'll now see Mixtable set up this new column with a heading (which you are free to change), and populating all the cells with any pre-existing data for the metafield you just set up.
Now, modify any of the cells with the values you want. In this case, we are setting up Product metafields for our products.
When you are done, click on the Done Editing button, and Mixtable will sync the metafield information to Shopify. It may take a few seconds before you see the new information in your store.