. You can reheat it using any of the following ways.
Set your toaster to Medium-Low, place the slice in, press down the lever and remove when done. Many toaster ovens now come with a "frozen" option to make your job even easier.
2. The Oven:
On a baking sheet, spread out a sheet of aluminum foil and place your slices on the foil. Preheat your oven to 375 degrees F, and place the sheet on a rack once the oven has come to temperature. Allow the slices to warm through for about 8-10 minutes. [Source
Coat a large baking sheet with cooking spray, then arrange French toast slices, leaving at least 1/2 inch between them. Place the baking sheet in a preheated hot oven, 400 degrees Fahrenheit, and watch the toast carefully, turning the pieces to crisp both sides. Frozen slices take about 3 to 4 minutes on each side, refrigerated slices less time. Use a convection oven option if you have one. The air movement in a convection oven distributes even heat that makes the coating crispy. [Source
3. The Skillet:
Simply warm the pan over low heat. Grease it lightly with oil/butter and place the toast. The number of toast at a time will depend on the size of your pan. Should take 1-2 minute per side.
4.The Microwave(least preferable option)
Reheating using microwave does not make for a crispy French toast since there is moisture loss/condensation that softens the food. You can minimize this by by layering a paper towel on the plate under the French toast, then cover with another paper towel. Microwave for about 10-12 seconds at a time and keep checking the toast. Once done, discard the paper towels and serve with your favourite additions. [Source