I bought a this Apple Watch series 5 from the previous owner.how can I use it without the Apple ID and password and Also I don’t have contact number for the previous owner. What should I do?

Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.

About Activation Lock on your Apple Watch - Apple Support

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. Try to get a refund (through eBay) or dispose of the watch properly. In the future, see the 'Before you buy a watch from a third-party' section in:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

You can't unlock it. Apple won't unlock it for you. We cannot help you unlock it. You are the proud owner of a useless phone, which will never work unless you can get the previous owner to unlock it for you. Since you don't have a way to contact the previous owner, you wasted your money and you most likely bought a stolen device.

If you can't reach the previous owner or get your money back, recycle the phone responsibly as it's useless to anyone.

Read this too --> The All Too Common SAD Reality of Buying … - Apple Community

Without the previous owner’s cooperation, the activation lock will prevent that device from being re-used.

See:

How to remove Activation Lock - Apple Support

Ps: I meant watch above, not iPhone. Everything else I wrote fully applies, however.