Electric potential energy of a system of point charges is defined as the work required in assembling this system of charges by bringing them close together, as in the system from infinite distance.

As we know,

E=-V

Electric field flows from high potential to low potential and a positive charge moves in the direction of electric field. In this process, it loses its energy.

But when a positive charge is moved from low potential to high potential i.e. in the direction opposite to the electric field, its energy increases as some work needs to be done to the particle. This work done gets stored as energy in the particle.