For Android phone, tap on the three little dots in the top-right. This will open the WhatsApp menu. From the menu that appears, tap on Settings. For iOS just tap Settings in the bottom right corner of your screen.
Select Account and then select Privacy.
Under Privacy, you should be able to see the Last Seen option and by default it is set to Everyone which means that everyone can see your Last Seen timestamp. Now we need to change the last seen settings. Tap on Last seen to proceed.
From the screen that appears, you'll be given three choices. Select Nobody and no one will be able to see your last seen in WhatsApp anymore.
If you pick this option, your last seen information will be visible to everyone.
Select this option if you prefer only your phone contacts can see your last seen information.
When selecting this option, no one including your phone contacts can see your last seen information. Please note that you will not be able to see your contacts last seen information as well.