Y Is it not changing

Generally these devices don't spend a lot on the battery and after hundreds of charge/discharge cycles, the battery fails. That's the most likely reason, but certainly not the only possibility. If it was mine, I'd open it up and check the charging circuit with a voltmeter. If it's producing power in the sunlight, then I'd leave it charging for at least an hour, then cover the solar panel and check the battery voltage to see if it's holding a charge.

Outside of those general ideas, we can't get any more specific without knowing more about your lamp. A manufacturer and model number or even a couple of pictures of the device might help us identify it and see if there's any information about it available online. You can edit your question to add information or pictures. This guide will help if you want to add photos to your question.

Adding images to an existing question - iFixit Repair Guide