You cannot modify your abstract's body nor title after it is submitted with no exceptions.
Until the 1st round abstracts submission deadline, you can delete the abstracts and still submit the new correct version. No need to inform the ERS office.
After you receive the notification that your abstract is accepted to the ERS Congress, you will be able to modify only the list of authors via myERS.
For further information about abstracts, we invite you to read the abstract FAQs.