If you have download products that aren't available for sending Updates, those products are likely configured with settings that are incompatible with Updates, as explained on our help page:
NOTE: Products using any of the following settings will not be available for sending an Update:
Send Stored/Generated Codes;
Send Transaction Data to a URL / Payment Variable Information URL;
Remote Product File URL.
You can send Updates for such a product by temporarily disabling those features in the product's settings, then re-enable them if necessary after sending the Updates.
If your different products actually provide the same download file, you can just select the main version of that product to send as an Update, then select the Buyer Groups for whichever original products' buyers should receive that Update (hold Ctrl on PC or Command on Mac to select multiple Buyer Groups).