For the most part, recipes are only ideas. Once they are in your possession they can change to suit your needs. I would say baking is the area that needs the most caution when changing, due to the way ingredients interact. Like amount of baking powder to flour, wet ingredients to dry, or number of eggs without making it taste eggy.
Whether you are missing ingredients, prefer others, or you don’t think it will turn out to your liking, you can change many things. Substituting oil for butter or another kind of oil, onions for shallots, or just switching up the veggies in your stir fry.
Cook times vary often. Not every oven, stove, and microwave work the exact same. The implement you use also changes the timing, like cast iron, clay, or other bakeware. Not everything heats evenly, nor fits perfectly. Also the amount of food you are cooking at once, spreads out the heat, increasing cooking time.