If you have any symptoms of anaphylaxis, stay where you are. You should lie flat with your legs raised to help blood flow back to your heart and vital organs.
If you are struggling to breathe, you may need to be propped up, but this should be for as short a time as possible.
You must avoid any sudden change in posture. You must not stand up, or sit in a chair, even if you are feeling better. This could lower your blood pressure drastically, causing your heart to stop.