Celeriac keeps well in the fridge for several weeks. If you use a portion of it, simply wrap the rest tightly and save in fridge. (You will need to slice off the outer edge as it will turn brown, but the rest will be fine!)
Awesome in soups, stews, roasted or mashed. Or steam until just tender, cool, and slice into julienne strips to make a celeriac remoulade (a simple one can be made with mayonnaise, mustard, and garlic...or add a bit of curry powder or chipotle if you like...fresh parsley too!).