If the glare has blown out detail completely, Photoshop can only do limited repair—it can’t truly recover information that was never captured. For an existing file, a retouching approach such as frequency separation may help reduce the shiny area by rebuilding color/tone separately from fine surface texture, but it’s a partial fix rather than a perfect one.

For a proper result, the best answer is to reshoot using cross-polarization. A common setup is:

• two lights placed at fairly wide angles to the artwork
• polarizing material on both lights, oriented the same way
• a polarizing filter on the camera lens
• rotate the lens polarizer until reflections are minimized or disappear

This is a standard technique for photographing artwork, especially reflective paintings such as oils. It helps suppress glare at capture, which is much better than trying to remove it later in post.