My requirement is how can I prepopulate the form details of previously completed tasks in the current task.
My assumption is that "the current task" is a subsequent task in the same process instance as the "previously completed task". Assuming this is true, then what I said above, and what Romulo demonstrated in their series of screenshots above, is applicable to your case. That is, the earlier task(s) will need to be configured to save their data / changes into the Process PV(s), and then subsequent task(s) will need to be configured to pass any necessary value(s) into their Task Forms.
The necessary configuration for this is of a fairly basic level in terms of learning Appian development, so I'd recommend you complete the Appian-supplied training course(s) on Process Model design as these concepts should be covered there. Have you explored any of the available online courses yet?