Lower sash in which the thermal seal has broken requiring replacement of the thermopane element with imbedded mullions

We cannot determine how tO remove this sash in order to have the thermopane section replaced. The white metal frame slides into side channels with no way of determining how to remove the sash--the upper and lower sashes are stacked over each other in the same channel. The label inside the window is"

Sponsored---Manufacturere Stipulates Conformity to---MFGR: TOM-U14-1---SECURITY TESTED
CAWM ANS/AAMA 101-88 SERIES: 88 etc.
Program DH-C20 MST: 4-6 X 7-6 CBA LEVEL
MVT: 52" X 43" in conformance to
ASTM E774-?4?