Cannot access my yahoo email account as my mobile number has changed.

[Bazza777]

2-step security is activated sending the code to my old phone number which is obsolete. 

[caesarome]

@Bazza777 

You will need to turn this off then set it up again with your new number,

To turn off 2FA sign in with the recovery code so at the login select "can't sign in with the code" link. You'll then be asked for the recovery code.

When you signed up to 2FA you were given a recovery code so if you didn't make a note of it then check the email you were also sent.

Help can also be found here on how to turn this off and/or change your number:
https://www.sky.com/help/home/sky-yahoo-mail/two-step-verif/articles/two-step-verification

[Mark39]

@Bazza777 if you don't have access to the RecoveryCode, you'll have to call Sky. That may be a challenge, as although there's a team that can deal with it, the front line adviser may not be aware of them.

[Bazza777]

No I do not have the recovery code. 

[caesarome]

Then you will have to give Sky a call as per  what @Mark39 has posted above.