You are still able to amend your 2017/18 return.
If you filed this online then you can go back into your account and amend this up to 31st January 2020.
Details on how to do this can be found here:
If you need to change your return
If you filed a paper return then you can write to us and tell us which boxes you wish to amend.
Please bear in mind that you cannot claim expenses along with the £1000 allowance.