Lost or forgotten passwords can be reset using this section on HMRC’S website
Click on the ‘reset your password’ link and you will be asked to enter the email address which you first used when setting up your Personal Tax Account.
If the address is correct, you will be sent an email from HMRC – firstname.lastname@example.org with a link so you can reset the password. You just click that link and then put in your new password, confirm it, and click Reset. You will then be redirected to another page where you can sign into your account.
If for some reason you don’t have your registered email address, you will be asked which kind of account you hold, ie are you an individual, an organisation or an agent, and then you will have to enter various personal details to enable HMRC to verify your identity. You can then reset your password and choose how you would like to receive it – either by email or by post.