To do so, go to your profile page using your profile icon in the right above corner of your Homepage. Here you can disconnect your account from your previous employer by removing it from the 'Your current employer' field. Also update your professional email address of your former employer by your new one or by your private email address. As a final step, do not forget to save the changes by clicking the 'Save my profile' button.
Remark if non-UK : just by adding my mobile number or by connecting my profile to the itsme app, you are assured of your access (even if you don't have access to your former employer's email address).