How to replace a sealed unit?

Assuming that the glazed unit has the push in retention strut outside -
First you need to remove the inner seal by pushing the window outward. Prize the seal up and remove all or it. Next go outside and with the aid of a paint scraper push inward and upward between the frame and the retention strut on the longer side. It will sort of ping out. Next remove the other struts in a similar fashion.
When you fit the new window do the opposite of removal.
If the retention struts are inside go outside first and remove the seal as above.

ray
September 2009