Wrong Insurance Number on letter

Hi. I got a letter with my upcoming tax code but I've just noticed that it has a temporary NI number and not my proper one. I've never paid much attention to the letters but I am pretty certain I've had this temporary number on previous years letters and want to know how I get this correct. I've the correct NI number on things like payslips and wondering how I can go about getting this corrected. Thanks.

Hi SBUK,
If you have a temporary National Insurance number HMRC will be advised of the correct number by our National Insurance team and we will update your record accordingly. 
Thank you.