Expat in USA: Cannot update surname and location before verifying my NI Number

Can anyone help, going round in circles. Can't access the options to update my (divorced) name and overseas address in order to verify my NI Number. Been away 19 yrs, Want to return soon -and want to purchase tax credits before deadline. Every attempt to verify my identity is failing. Thank you, SH

Hi Sheril H Holbrook,
The best way to notify us of multiple changes to your personal details through the post.
You can write into us at:
PT Operations North East England
HMRC
BX9 1AN
Please include your NINO, full name including middle and any previous name, current address, and any previous addresses, date of birth, please sign and date your letter even if you are typing and printing it out.
Because you are changing your name, please include certified copies of documents confirming your new name.
You can find a list of documents that can be used and how they can be certified by searching for Certifying a document on gov.uk
Certifying a document
Thank you. 
 

Thank you, this is SO helpful! If I get that documentation in the mail from the USA, tracked/registered post, Is there any way I can find out how long it can be changed- trying to not miss the NI contributions buy back deadline! Thank you again!