I am currently using Adobe Photoshop CS5. I have a png file with some signatures and their texts (names of the persons). I have to change font of the text, but I haven't seen any of Text Layers, only Background that seems is locked. How can I change the font of the text? Thank you in advance for any help.
If it's a .PNG file most likely its a flat image, and has no Photoshop layer data in it, therefore the text and all the other elements in the image are rasterized.
The only way to edit the text is to remove it and retype it again using the font you want.
There is a tiny chance that the .PNG was saved in Adobe Fireworks. Try opening it in Adobe Fireworks, there is a slight chance it might contain Fireworks layer data.