How do I amend last year's EPS because I should have claimed Employment allowance but didn't?

Hello NatalieDawn, thanks for posting on the Community page, If you are using advanced payroll as long as you tick the box and then save then go to report in payroll then setting EPS option for hmrc reporting then check the year and then submit eps and then chose the date you want the information sent and then click submit and then in reports check the P32 . 

If your using standard payroll if it is for previous years you will need to do it via basic tools. Here is the link for basic tools https://www.gov.uk/basic-paye-tools . If it is for last year before you would need to roll back on standard payroll to the point of needing to turn on then turn it on send that eps notification and then roll forward as a correction. This article tells you how to roll back/delete a payroll.