How does work change the energy of an object?
The energy transferred as a result of work being done can be either positive or negative. For example, in the mechanical sense, work can be done by an applied force to increase the velocity of an object. Work can also be done for example by friction to oppose the motion of the object and decrease its velocity.
In either case, the amount of work done in Joules is equal to the change in mechanical energy brought about.
The work done on an object might also result in an energy change outside the actual mechanical system. For example, the mechanical energy may be converted into an equivalent amount of heat energy, light energy, sound energy, energy of deformation, etc as a result of the equivalent amount of work being done on the object.