when a force of constant magnitude always act perpendicular to the motion of a particle then the motion should be uniform circular motion therefore direction keeps changing therefore velocity also changes.then how can the kinetic energy be constant

Dear Student,
When the particle is acted by a constant force perpendicular to the motion of the particle then the motion of the particle is uniform circular motion so the velocity of the particle changes with the change of direction but the magnitude of velocity remains same here .
So the magnitude of velocity only changes but the magnitude remains same which is the main reason that  the kinetic energy remains same .
Regards