Steps to  hide last seen on Whatsapp:

• Simply launch the WhatsApp app and navigate to the Settings area to hide your last seen.
• Now go to the Account area and select Privacy from the drop-down menu. It's worth noting that whatever changes you save will apply to the messaging app's mobile and web versions.
•   Toggle the Last Seen option to 'Nobody.'
• You have three options: 'Everyone,' 'My Contacts,' and 'Nobody,' to name a few. The first simply implies that anyone with your WhatsApp number can see your most recent location. Only your contacts will be able to view your most recent location on WhatsApp if you choose the second option. Nobody will be able to see your WhatsApp last seen if you choose the 'Nobody' option. At any point, you can choose any of the last seen options.

Read More: How does Whatsapp work?