Saving a dying spruce tree means identifying the ailment and treating it accordingly. The spruce might have a fungus, an infestation of beetles or might simply need improved living conditions.
A common ailment for spruce trees is the cytospora canker, a fungal disease. It is especially severe on Colorado and Norway spruces. Individual branches start dying, especially the oldest ones. White residue and cankers can be seen on infected branches. Cytospora canker manifests during drought stress, so infected trees need individual irrigation. Prune away infected branches. There is no chemical control for cytospora canker, but trees can live for years if the drought stress is alleviated.
Another culprit might be spruce bark beetles. In this case, the tree starts yellowing and dropping needles. Burrow holes about 2 millimeters in diameter in the bark indicate the presence of beetles. Drought stress is a culprit here, too. Uninfected or partially-infected trees can be sprayed with pesticides. Fully infected trees need to be cut down to avoid infecting neighboring trees.
If the spruce trees are in close proximity to each other, they may be competing for sun and nutrients. If the trees shade each other, they may start dropping needles. If that is the case, the weakest trees should be cut down to allow the others to thrive. Alternatively, smaller trees can be replanted elsewhere.