They need to declare bankruptcy, but the UAW is lobbying hard that they don't. Once the bankruptcy is filed, any and all agreements with the union are subject to renegotiation, up to and including termination.
The GM/UAW model is woefully outdated, as evident by the non-union shops that produce cars. The UAW is an ominous and overbearing entity that believes it has to be present to sustain the American auto market.
Well, it doesn't.