To know the Cashback offers in Paytm, you have to log in to your account. Once the page gets loaded, you will find a search box at the top of the page. Just to the right side of the box, there is the CashBack Offer written in blue fonts. Click it, and you get to know the currently running offers on Paytm. Moreover, Paytm also alerts you with notifications. So, if you click on the Notification Bell, placed right near the search box, you will find many of the cashback offers they have notified you about.
All you have to do is, activate this offer by scrolling down and pressing the button for it.