Microwave the spaghetti in a microwave-safe bowl
Add a splash of water, broth, or sauce
Cover loosely with a microwave-safe lid or damp paper towel
Heat in short intervals, stirring between each
Stovetop: place spaghetti in a skillet or saucepan
Add a little water, broth, or sauce
Warm over low to medium heat, stirring often
Oven: place spaghetti in an oven-safe dish
Add sauce or a little liquid to keep it moist
Cover with foil and heat until warmed through
Reheat only the portion you plan to eat
Stir and check that it is hot throughout before serving