Why it is said that during uniform circular motion direction of acceleration is towards centre but not constant????
Please explain in brief.

Dear Student, 

Acceleration is defined as change in velocity with respect to time we know that velocity is vector quantity which has both magnitude and direction. In uniform circular motion the acceleration is produced due to change in the direction of velocity continuously. Hence, the acceleration in uniform circular motion is variable and not constant.  
The acceleration towards centre in uniform circular motion is called centripetal acceleration. 
​​​​​​Centripetal acceleration is the acceleration a particle must have to follow a circular path. This acceleration always points towards the center of rotation and has magnitude 
a_c = v²/r
v = velocity 
r = radius of circular track 

The centripetal acceleration is directed along the radius in the circular motion.
Regards