Tax code error

How do I change the default tax code used when calculating a self assessment submission? My tax code should be 1382M and is shown elsewhere and on my P60 but is not used when calculating my tax from my self assessment submission leading to an incorrect tax quote.

Hi StillTheSame,
If your tax code is 1382M then you will be in receipt of the marriage allowance transfer. When you submit your Self Assessment the marriage allowance transfer will not be shown on the calculation but once received the system will pick up that you are in receipt of the transfer and the calculation will be amende to include this.
Thank you.