What can I do to help my baby banny chicken get better?

Sounds like it's in distress or overheating.

If you have a light over the box/cage they're living in make sure it's not directly below the light. It needs the warmth from the light but too much will make it overheat so put it in the medium/warm area of light (use your hand to figure out the correct amount of heat) If you don't have a lamp over them you need one. Otherwise they'll get too cold.

Make sure the other chicks aren't running over the top of it. Otherwise they'll crush it.

Feed it more water. It's easiest to grab a shallow lid (like one for a ziploc container) and fill it with a little water. Dip the bottom edge of the chicks beak into the water. Usually they'll open their beak and drink. It makes it a lot easier to feed them that way as they'll not make a mess trying to fight you. One hand holds the chick the other holds the lid so you can tip the lid when the bird starts to drink.

Give it lots of attention. Make sure you have someone who can check the chick while you're gone.

Make sure you feed it cut up worms or chick feed mixed with a little water. You can add a tiny amount of fruits. Cut up Grapes are good because of the natural 'wormy' texture and the natural sugar gets the bird going.

Call your local vet...they may not treat chickens but they'll usually give you a mix to put in the birds water or feed to boost it's immune system and help it. And the mix is usually not that expensive.

Once it starts to get it's strength back you'll want to do some physical therapy (sounds weird I know) straighten and bend it's legs gently a couple of times to stretch it out. Use your hands to hold it gently over the ground and balance it while letting it hold a little of it's own weight on the ground. After awhile it will have the strength to stand.

There's a good chance your bird will survive but there's also a chance that no matter what you do you'll loose it.

Good luck!