how do i erase all content from my watch if i already unpaired it?

Hi there, dishaila!
Thanks for reaching out to the Apple Support Communities! That is a great question you have about erasing all content from your Watch and I am happy to help out! Since you have sold the Watch and no longer have it in your possession, you can check out this article for iCloud: Remove your device from Find My iPhone. Specifically:

Remove an iOS device, Apple Watch, or Mac you no longer have

If you no longer have the iOS device, Apple Watch, or Mac because you gave it away or sold it, you need to remotely erase it before you can remove it. With Apple Watch, you must have watchOS 3 or later installed to remove Apple Watch.

1. Click All Devices, then select the device you want to erase. To erase:

   • An iOS device or Apple Watch: Enter your Apple ID password or your family member’s Apple ID password. If you’re not using a trusted browser, answer your security questions or enter the verification code that is sent to your other devices. If you’re using your own computer and want to skip the verification step in the future, click Trust. If you’re using a friend’s computer, click Don’t Trust. If the device you’re erasing has iOS 8 or later, enter a phone number and message.

   • A Mac: Enter your Apple ID password or your family member’s Apple ID password. If you’re not using a trusted browser, answer your security questions or enter the verification code that is sent to your other devices. If you’re using your own computer and want to skip the verification step in the future, click Trust. If you’re using a friend’s computer, click Don’t Trust. Enter a passcode to lock the Mac (you need to use the passcode to unlock it), then enter a message.

   If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.

   Any credit or debit cards set up for Apple Pay on your device are removed from your device if you erase it, even if it’s offline. Apple Pay is also disabled for your device. For more information, see the Apple Support article Use Wallet on your iPhone or iPod touch.

2. When the device is erased, click Remove from Account.

   All your content is erased, and someone else can now activate the device.