Your sterilizer will come with detailed cleaning instructions, but as a rule, most removable parts of any unit can be washed with mild soap and water, while the rest can be wiped down with a damp cloth.
Limescale build-up looks like a yellow or white discoloration on the sterilizer’s heating pad. This can be removed at home with ordinary white vinegar or even lemon juice: simply soak for 30 minutes to overnight and rinse off well, followed by an empty cycle to dislodge any residue.