The easiest way to manage your PAYE code is via your HMRC Personal Tax Account. In your Personal Tax Account, you can review your current and future PAYE codes; update your expected level of employment income; add or remove non-cash benefits (private medical insurance, for example) or pension contributions.

Waiting times on HMRC’s phone lines can be upwards of one hour. As such, you may find it easier to use HMRC’s online chat service which can be accessed via the “Ask HMRC online” link on this webpage.
The webchat does begin as a robo-chat but, with some persistent questioning, you will eventually be queued to chat with an HMRC agent.

Where individuals earn above £100,000 in any tax year, the Tax Free Personal Allowance (the first £12,570 of income that is taxed at 0%) begins to reduce down as income rises above £100,000.
HMRC’s information on this matter can be found here.
If your PAYE code was 1257L for a tax year - that is, a PAYE code that allows the personal allowance in the calculated payroll amounts - but you earn over £125,140, you will owe £12,570 @ 40% = £5,028 due to the under-withholding of PAYE.

Tax relief on pension contributions is a complex area.
UK pension contributions may be made to pension funds on a pre or post tax basis.
In certain circumstances, UK relief may be available for contributions to non-UK pension schemes.
If you are a PAYE only taxpayer who is not within the scope of Self-Assessment, you may need to adjust your PAYE code for ‘Relief At Source’ pension contributions.