Hi,
The rate of national insurance is determined by your national insurance category letter on your payslip and the gross amount of earnings you receive in your pay against the frequency you are paid.
You can check your payslips using the national insurance calculator
Work out an employee's National Insurance contributions
The national insurance categories rates are listed for your scenario here
Rates and thresholds for employers 2023 to 2024
If you employer has made the wrong amount of NI to be deducted, they should re-imburse you and amend any records sent to HMRC.
If they are unwilling to do this, we can only look at your pay after the financial year has ended and has been reported as whole.
Thank you.