How to redirect user to thank you page after the paypal payment is completed?

And if someone pay me through the Paypal, then how they can be redirected to the THANK YOU page from paypal?

As , they will be in PayPal page at the time of payment. Do I have to change some settings ?

And, If I dont change the IPN URL to : https://www.jotform.com/ipns/paypal.php ; 

then JOTFORM will not get any notification from paypal by default ?

And if someone pay me through the Paypal, then how they can be redirected to the THANK YOU page from paypal?

You just need to setup your form's Thank You page. Once the user has successfully paid from Paypal page they will be redirected to the Thank You page setup to your form. For more information, you can check this guide: Setting up the Thank You Page.

And, If I dont change the IPN URL to : https://www.jotform.com/ipns/paypal.php ;  then JOTFORM will not get any notification from paypal by default ?

No. The default notification URL does not matter. JotForm just needs IPN to be enabled. You can enter any Notification URL, but for JotForm-initiated PayPal transactions, the URL will always be overridden with https://www.jotform.com/ipns/paypal.php. You can check these guide for more information:

How to Enable IPN on your Paypal Account

Paypal Incomplete Payments FAQ