You can export an IFC file by going to Models (1) and then choosing the Exports (2) button.
On the next page, you can see the history of previous downloads and redownload them individually.
If you want to merge several models into one file, a "New export" must be created.
The next step is to choose the export options.
Choose whether to archive the the file as a .zip or .xls file
Then choose which models and which revisions of those models will be included in the export.
To export, scroll down the page and find point number 6.