I have a client application now where users open word documents using ms word and save their changes. Now I am going to create a web application with familiar features. I would like to open the word documents in word online and save their changes back to lotus notes document. How we can achieve this? I don't have any clue where I need to start.
Several ways, but not in a web form unfortunately:
You can copy the file to Microsoft OneDrive, Dropbox or Google Docs, and use their Office-tools
There are other packages that you can install on your server that allow your users to manipulate .docx files
Actually, we use Swing Software in our web application to download and upload Word files from a Notes database so the user can modify the file on their PC (using Word or Libre/OpenOffice). That may not exactly match your requirements, because Word has to be installed locally.