It is bent this way due to the way they packed it in a box. What should I do about it? Should I try to bend it other way manually or let it get straight by putting in sunlight
Just put it in the light. It will react accordingly. Here is an article that will help you to care for your plant:
When I shifted a bushy , green arrowhead plant from soil to water, the leaves have started turning brown n the overall plant looks dull n sad. The pot is placed in bright indirect sunlight.
Are you trying to grow it hydroponically? If so, there is much more to it than putting it in water. Here is more:
https://www.gardeningknowhow.com/special/containers/hydroponic-gardening-indoors.htm
https://www.gardeningknowhow.com/special/containers/hydroponic-water-temperature.htm
Its a arrowhead plant. T.Y.?.
To root plants, they need bright light but it doesn't have to be a grow light unless you are propagating in a dark room. Setting them near a window is fine (but out of direct sun). Arrowhead plants are typically listed in the low light needs category.
If you are using lights, the article below should help:
https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants#factors-to-consider-when-you-evaluate-light-2651611
I've grown arrowhead plants for years. I've never had this happen. My plant seems full and healthy. It appears to have a lime green mushroom in it.
It means fungi have gotten into the soil either through the potting mix or by air. They won't hurt you or your plants. Just don't eat them. Here are ways to combat the problem: