Prepare the pumpkin and lemon. Cut the pumpkin pulp into small pieces of 1 cm. Cut the lemon into small slices together with the peel. Mix the pumpkin, lemon and sugar. Leave for 3 hours.
Cook the jam. Add water to the sugar-pumpkin mixture, put on medium heat. Bring to a boil and cook for 1 minute. Cool completely for 3 hours. Bring to a boil again and cook for 3 minutes. Cool the jam completely for 3 hours. Bring the jam to a boil again and cook for 5 minutes. Add cinnamon and stir well.
Preserve the jam. Place the hot jam into sterilized jars. Seal tightly with lids.