Hello Yes .. you can always tell your dentist that you just want to remove the upper one

If u don't want to remove the lower one ,u cn tel him to remove just upper one ,lower one u cn remove if you get any discomfort later,

Once your upper tooth is extracted the lower one is not of much one as it is not biting with any other tooth so it will not help you chew. Besides after sometime it will supraerupt( go above the occlusal level) causing food impaction. Hence your dentist is advising you to remove both. However if you do not want, you may remove only the upper one. Normally extractions will be painless as your dentist will give you a local anaesthetic.