It is of course your choice to determine if her quality of life is suffering and if it would be better to end it for her. There are several methods of euthanasia:
1. Freezer. Extreme cold will knock a mantis out rather quickly but you need to leave her in the freezer for 24 hours because there have been cases of them waking back up after only spending a couple hours in there. This is the preferred choice among most of the keepers here.
2. Crushing. Place her between 2 pieces of paper then use a heavy book to quickly, firmly and decisively crush her, it has to be hard and fast. This is a quick way to end her suffering but not everbody has the wherewithal to do it this way.
3. Carbon dioxide from dry ice. Put dry ice in a small container with a lid until it completely sublimates then place the mantis inside and replace lid. Leave her in there for at least an hour. This method is commonly used to euthanize vertebrates. I have used this method once with success for a mantis but this
wiki on euthanizing invertebrates suggests that this method can possibly cause spasms and not much is known about the effectiveness.