How do I export solution from 2016 for 2015

I have a solution with customizations I have been working on for a while in my CRM. Unfortunately it was upgraded to 2016 in the middle of my customizations and the client for whom I made the customizations is still (and will remain for a while) in 2015, so now I can't import my solution into the client's CRM.

I am a bit desperate as I have invested a lot of hours and have a deadline of tomorrow (today really). Any help will be very much appreciated.