Celeriac can be left in the ground until you want to use it, although you will need to protect it against hard frosts. Alternatively you can pull it up and store it now before it turns too cold.
If you’re growing celeriac in a fairly mild, sheltered area, then it is hardy enough to stay in the ground through the winter. You’ll want to pull some straw or loose compost around the crown of the celeriac roots though, to protect them from frost and snow should we get a blast of cold weather like we did last year.
If you’re in a more exposed spot, then it is advisable to pull them up now, towards the end of November before temperatures get much lower. Whilst they are hardy, they will struggle in severe cold frosts/snow. You can store them quite well in a sack in a cool, dark (but dry) place.