CPV code 45259900 designates Plant upgrade work in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
45259900 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 45259000 (Repair and maintenance of plant).