Three ways, depending on what you have to hand.
You know the current password: log in to webmail (https://yourdomain.co.uk:2096), open Settings → Password, and set the new one.
You don't, but you have cPanel: cPanel → Email → Email Accounts → Manage next to the address → enter a new password under Security → Update Email Settings. No old password needed.
Neither: open a ticket from your client area naming the address and we'll reset it to a temporary password for you to change in webmail. Please don't put a password you intend to keep in a ticket or an email.
After any change, update the password on every device that uses the mailbox straight away — a phone still trying the old password will fail over and over, and the server firewall reads that as an attack and blocks that connection for everything, including your website. If that's already happened, see Email or website works on mobile data but not on my Wi-Fi.
