There are many posts about this.
The following assumes you haven't used any new features that didn't exist in the previous version.
If you are saving as a Project, then edit the .caproj file using a text editor and edit (roughly line 10 I think) the following line:
Alter the 17100 to 17000 and save. You should then be able to open it with r170.
If you have been saving as a .capx, just unpack the .capx using Winzip or 7zip, then follow the above instructions.