As I understand it, HbA1c tests indicate the quantity of glycated haemaglobin sloshing about in your bloodstream. This increases as the level of glucose in the blood increases. Once the glucose has bound to the haemaglobin, it stays bound and therefore is a good indicator of blood sugar over the previous few months.
If this is the test you're having, doing a bit of exercise before the test or cutting down on sugar for few days beforehand won't make any significant difference to the result.
I had a similar problem a while back. Stopping drinking full-fat Cola (I was a demon for drinking lots), reducing my chocolate intake and using pretend sugar in my cups of tea not only meant my blood sugar level soon returned to a normal value, but also lead to me losing a stone and a half. I feel much better for it and I no longer crave the sugar rush!