I would like to add a video to a description of a session. The video is upto 700 MB. How can I do the same?

The upload limit in the editor field is 10MB. 

If you wish to upload a larger file (up to 700MB), you can either upload it under Pre-recorded session.

Or if you have multiple files to upload, you can upload it under the respective speaker's abstracts and then in the sesion editor, select that video under the session where this abstract is added as a sub-session.