Lot2024

Hi, needed some advice, so the last 2 financial years i have done my returns via an accountant and this year i though id save money by doing it myself. And i guess i should’ve got my accountant to do it as ive now got an eye watering tax bill as ive declared my income way way more than i thought it was. I should’ve done Apr 23 to Apr 24. But i accidentally included income all the way up to this month in the calculation. Meaning i did 19 months worth instead of 12 months. Am i able to amend it simply online? Will i need to send in evidence as i will be putting in my actual amount of income? In case HMRC think im trying to lower it on purpose. Very anxious and confused. Thank you for any advice.