Do keep in mind that products uploaded via CSV are not synced with Modalyst, and you would need to update your store yourself.
Here's how you can download your CSV file with Modalyst:
Add products to your Import List
Go to the Import List
Select the item or items you want to export
Click Export to CSV
Export your list as a Generic CSV file (or the Shopify CSV)
Check your e-mail to download the CSV file
Please note: Once you've downloaded the CSV file, you would need to re-arrange the columns to match the format set by your shopping cart software in order to upload all the products at once.