You will find the export button when you are in model viewer with task menu open. The button is located in the top right of the screen. By clicking the export button it will open the export screen.
The export screen gives you the option to download your model to your local hard drive. You can either download a single model or a zip-file consisting of separate (not merged) models. You can also choose to bake into the exported IFC file(s) tags and properties (library contents, bsDD or user defined) that you have added using the viewer.
New export:
By clicking the new export button in the top right of the screen you will open a new window. From here you follow the steps to make a new export:
Format options
Select model(s) and Revision(s)
Merge options
Add the name and comment of the project
Sharing options
Press the export button