Fill a large bowl with ice water and set it near the stove.
Bring a large pot of water to a boil.
Cut a shallow X in the bottom of each peach.
Drop the peaches into the boiling water for 30 to 60 seconds.
Remove the peaches and place them into the bowl of ice water.
Peel off the peach skins, remove the pits, and chop the peaches.
Add the chopped peaches and lemon juice to a medium saucepan or Dutch oven.
Cook over medium heat, stirring occasionally, until the peaches release their juice.
Once the mixture begins to bubble, reduce the heat slightly and simmer for about 10 minutes.
Mash the peaches with a potato masher or immersion blender to your desired consistency.
Stir in the granulated sugar and cinnamon, if using.
Cook for another 5 to 15 minutes, stirring often, until the jam looks glossy and thickened.
Test the jam on a cold plate. It should hold a soft trail when you run your finger through it.
Remove the jam from the heat.
Spoon the hot jam into clean jars.
Cool to room temperature before covering and storing in the refrigerator or freezer.