how to change client side validation text message

i want to change text message of the  default validation message that occur at client side 

by default the client validation for text boxes shows required field. I want to change that message to something meaning full like " Please enter Task Name"

Hi Tarun,

Assign the  "Please enter Task Name" on the Textbox1.ValidationMessage and Textbox1.Valid = False inside the action that triggers the validation.