Hello Isobel
Currently there is not a way to export the sales invoice to word from the desktop product..The DES file may be able to be converted using a third party app.
You can can create a new form in QuickBooks Online in the cog in the top right> new style>custom form styles>import style>download a sample>invoice samples and help.
You can either use one of the samples and change the logo,address,business information etc and fields to what you want or use the help to create a new form in word and upload it to QuickBooks Online
Thanks
Emma