Hi @Murad Shalbak, welcome to the forum. For which modem do you want to change the password? The process isn't the same for each model.

The V10 and lower numbers start of without a password, the first time you log in you'll need to create a new password. The V10a and Box 12 have the admin password on the back of the modem.

If you don't remember the password that you set before, you can reset the modem. That way it goes back to the default option I mentioned.

I have Box 12 but the admin password on the back of modem doesn't work . You are suggesting that I do a reset the modem. but that will take away my internet connection configuration from KPN. Is this correct?

If you reset the modem the configuration stays intact. It will return the modem to the default settings it was delivered with, including the password.