To convert existing convertible securites to stock, head to the Cap Table tab on the left panel. Click Convertible Securities from top. Here, you can choose to Convert All to Stock or convert individually by clicking their Cert number to access the instrument's profile page.
Note: "Mark as Converted" will only show that the instrument has been converted as of a certain date, but will not create a stock grant for the conversion.
"Convert to Stock" will allow you to enter the number of shares the security will convert to and will create the stock grant in the selected stock class.
To learn more about note conversions, please see, "How much Equity is a Convertible Note Worth?".