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Most of the Divas exercises start at A3, and do mostly go up to C6, some up to D6.
I think track 8 goes up to D#6 at one point, but most of the scales start out on an A.
Use that feeling to know when to back off. You should stop short of any pain. With a bit of exercise stopping short of any threshold of pain, you should gradually be able to add a note or two or three, and eventually be able to sing those same notes without any discomfort.
Give yourself time to adapt to the higher parts of your range. It's slow-going, but worth the patient effort.