Unfortunately OP, you cannot simply get the DNAR revoked on your say so. However, if you have concerns about your mother's care I would advise you to talk to her healthcare team about them.
I think it is important to realise is that resuscitation is something that is a medical treatment and a patient/family cannot oblige doctors to attempt resuscitation if they feel it is unethical/futile or actively detrimental to the patient. Of course, in reality most doctors will want to work in conjunction with patient to help them understand the implications of both courses of action and will want to respect a patients wishes (which is the right thing). But at the end of the day it is a medical decision whether to attempt resuscitation and when to stop.
Resuscitation is not like you see on the TV. It very rarely works in frail people who have multiple serious co-morbdities, especially if cardiac arrest has occurred out of hospital. It is a brutal, invasive thing and should not be done in cases where it is highly unlikely to be successful. In these cases there is a low chance of success and an almost certainty that there will be some harm done. From experience, it's awful. In most frail elderly, if you are successful then there are usually complications and many will have a further cardiac arrest and pass away within a short period in any case. I'm sorry to be blunt, but it really is something I feel strongly about having seen too many very sick or frail people be put through a resuscitation attempt.
If your mum has capacity, it will be her decision in conjunction with her doctors. You can discuss your concerns with them but ultimately if she disagrees with you then your wishes will not supercede your mothers' decision.
If she doesn't have capacity AND you have power of attorney (welfare) you can discuss it with her doctors and they should work with you to make a final decision.
If she does not have capacity and you haven't got power of attorney, her doctors may well discuss matters with you but you won't necessarily have the final say. In my experience, someone who lacks capacity due to cognitive impairment and who also has several co-morbidites- one of which is a significant AAA- is highly unlikely to be a good resuscitation candidate. Any attempt would be futile and may actually lead to a very undignified death, as opposed to being made comfortable and having a peaceful passing.
I'm sorry if that seems harsh. I think you need to be very clear as to the reasons behind the DNACPR decision (and whether the decision is made by your mother) before trying to have it over-turned.