How to reset a password from kwallet?
I've read about a kwallet GUI that could be installed from synaptic, but I found nothing. Actually, it shows a GUI asking me the password, so is must have somewhere where I can change this password..
If you do not know the current password, it is not possible to "reset" it (this would be a security problem). It is only possible to remove the entire wallet and then kwallet should ask you to create a new password.
In order to delete the wallet, delete the file
If you do know the current password, then see Vaibhav Kaushal's answer below.
To change the password for KDE wallet, you need to first have it running. It it is not running, start it.
In the system tray, you would find its icon. Click on the icon (left-click, not right-click). A new window would open with the list of wallets (by default there is just 1). Right click the wallet and you have the option to change the password.
TO delete the wallet, you can follow what txwikinger said.