It's definitely starving and probably thirsty so take it out of its pot and give its root balla good soak in a larger bucket of water then you have a choice.
Plant it out in the ground at the same depth it was in the pot and with plenty of good compost worked into the soil. Give it a good drink after planting and every week this year till the autumn rains arrive. Mulch the roots with some well rotted manure or garden compost every autumn, after some heavy rain. this will keep it healthy.
Plant it in a much bigger pot with good quality compost such as John Innes No 3 and then give it a good drink. Make sure it is stood on feet or bricks to let it drain. It will be entirely reliant on you for food and water in a pot so give it regular watering so its compost doesn't dry out and a liquid feed once a week from early spring to the end of June. Tomato food or a seaweed mixture will do.
You can make jelly from the apples so it's worth looking after it and insects such as bees and hoverflies will love the blossom.