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You should leave them as they are until you are ready to harvest.

This vegetable is a heavy user of nitrogen. You should use a 30-10-10 fertilizer according to package directions.

This article will help with that: https://www.gardeningknowhow.com/edible/vegetables/turnip/turnips-growing.htm

This happens to turnips when they are left in the ground too long. The best way to avoid it is to harvest the turnips a little bit sooner. Here is more info: https://www.gardeningknowhow.com/edible/vegetables/turnip/harvesting-turnips.htm

The following articles should be of some help to you: https://www.gardeningknowhow.com/edible/vegetables/turnip/turnips-growing.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/what-is-bolting-what-it-means-when-a-plant-bolts.htm

Turnips need somewhat cooler temps and loose soil to properly form a bulb. If you are noticing more top growth and little to no bulb formation, it is likely due to a fertilizing/nutrient issue. While a soil test would allow you to know for certain, it may be that you have too much nitrogen and too little phosphorus. Lots of nitrogen will result in plenty of leafy growth but if your soil is lacking phosphorus, there will be poor root growth. I would add some more phosphorus, like bone meal, to the soil and give it some more time. Also, make sure the plants are adequately mulched to keep the soil cooler. Generally, it takes about 45 days from planting to see good-sized turnips for harvesting.