To begin with, there are actually a couple of ways to unlock layers in Photoshop.
Note: All screenshots are taken from Adobe Photoshop CC 2021 Mac version. Windows or other versions can look different.
The Easy Solution
I assume you all know that you can unlock the layer by clicking on the lock icon next to the layer in the Layers panel.
After clicking on the lock icon, your locked background will unlock and become a layer.
The Second Method
Another way to unlock a layer is simply double click on the layer. A box will appear where you can name the layer or choose layer style, click OK and your background layer will be unlocked.
Can’t Find Your Layers Panel?
Go to the top menu and select Window > Layers, and the panel should appear on the right-hand side of your document.
If you have tried both methods above and couldn’t solve the problem or your lock icon is greyed out, you might want to take a look at the document mode because some modes such as Bitmap, Multichannels, and Indexed Color don’t allow you to unlock or even add new layers.
There’s an easy solution to change the color. Simply change the document color mode to RGB, CMYK, or Lab Color.
You should get your problem solved using this method. Good luck!About hannah