Ideally, you install the Tech Preview on a whole other drive, or partition.
Lacking that, create an image of your current install. CloneZilla or DriveimageXML would do this. Save it on a whole other drive.
Then install the Win10 tech preview.
Note - this tech preview is NOT meant to be a primary OS, but rather a preview, just to see what they are coming up with and to provide feedback to MS.
Functions can and will change randomly, it may crash badly, your hardware may not support it, etc, etc, etc.