Hi There,
I had the same problem. You press the black button WHILE THE MACHINE IS OFF. The head pops off, leaving hte blades exposed. Then rest the new head by the bottom of the blades. Click the two together. (don't forget to put the bag on if the Garden Vac is in SUCK mode!)
THE MANUAL CAN BE FOUND HERE
https://www.2helpu.com/PDMSDocuments/EU/Docs//docpdf/gw250_uk.pdf
Good Luck!
Mark
Mark Sweeney, November 2008