1. As per existing practice, last year too, the CBSE offered an online option for verification of marks and also a chance to get photocopies of the answered booklets. This year the dates are not yet announced.
2.The CBSE offers re-totaling or re-checking wherein the marks total is re-checked for any calculation mistakes.If any question is left un-evaluated, then this will be evaluated. There is NO - RE EVALUATION of already checked answers, so if there is any arithmetic/human error in adding up the given marks or entering the final marks, then there is a chance for the marks to change.
3.Usually the dates for re-checking are annouced soon after the declaration of the resutls, from the annoucment date of verfication, 21 days are given to the candidates to submit the forms to the respective regional CBSE offices.
4. The fees are around 200-300 rupees per subject. Once applied, you can check the status online and the results are given within a months time.
5. If the candidate is still not happy with the results, then in the following year (2019 in your case) they can apply for 'Improvement exam'. This can be done in one or more subjects.