Hi, few days ago I saw that in my Yamaha P-45 pressing sound of C4 is different. There's a link below to video with the sound. Can I do something to fix it?
If you're talking about the actual sound produced through the speakers, you can do a factory reset on the keyboard by holding the highest white key (C8) while the piano is off, pressing the power button while continuing to hold the key. The power light will flash a few times to indicate you have restored factory settings.
That being said, your video seems to imply that you're concerned with the sound of the physical key hitting the key bed. I honestly have no idea if you can disassemble these keyboards, and with an entry-level P-45 (no offense, I have the same one), I think it's a safe bet that you're stuck with that sound. My lazy solution? Try to hit it with a vacuum and see if anything comes up, then maybe clean the sides of C4 along with the keys next to it.