You can use Mixtable's spreadsheet interface to easily add or remove Shopify products from collections. You can do this for individual products, or you can add/remove products from collections in bulk.
To add/remove products from Manual Collections you first need to map a column in your Products-linked worksheet. Right click on the 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.
From the column options you see, select the Product's Manual Collections radio button, and then click on the Save button.
You'll now see that the new column is set up to show each product's Manual Collections, and it will begin to populate with the handles of any existing Manual Collections that each product is in.
Now that your products' collections are mapped to a column, and you can see the existing collections that a product is in, we can add/remove products from Manual Collections as needed. One way to do so is with the Collection Picker. In your worksheet you'll notice that all the cells in the new, Collection-mapped column contain a button with a + symbol in them. If you click this button you'll open the Collection Picker widget. The Collection Picker allows you to visually add or remove a product from a collection. With the Collection Picker active, select a collection's checkbox to add a product to that collection, or uncheck a checkbox to remove the product from the collection.
When you are done click the Update cell with selected collections button. This will update the relevant cell with the handles of the collections that you've chosen for the product. Please note - even though you've updated the cell's value, these changes won't sync to Shopify until you press the Done Editing button. This allows you to make changes to your worksheet without sending them to Shopify, and making the sync only when you are good and ready.
When you've mapped your products' collections to a column, you'll notice that each cell now contains the handles of the Manual Collections that a product is in. These cells behave just like any other cells, and you can edit them manually or set their value to a formula. If the handle of a collection ends up in the cell then the product will be added to that collection when you click on the Done Editing button.
This behaviour allows you to easily modify the collections of products in bulk. You can use Excel-style formulas to set the collection handles for a number of products. You can also copy/paste collection handles from a cell to other cells to set the collections of a number of products in a single go.