Hi there Coco, sorry bro but that's not a sun burn, that's a nutrient burn from over feeding a too young seedling. At that stage in soil, you dont need any feed for the first few weeks (minimum three or even four weeks). The outside sun probably helped as it probably dried up the soil a bit quicker than inside which is a good thing for soil grows. As you can see, her leafs and cotelydons are drooping downwards, that's a symptom of root problems probably due to the nutrients and lack of water flow in the cup. in other words due to over watering or too much moisture for too long. Unfortunately the honest truth is that your seedling is stunted for ever now because it got affected so early on in life. If at all possible i would start the grow again, making sure you have little holes in your cup for the water to drain, and make sure you dont over water using the lifting method, sticking to a strict wet dry cycle. to answer your question, you do want to transplant when the seedling is big enough, that means that it has a large enough root mass, usually at the end of week three or even four. Hope this helps! 🚀