How can I bypass the activation lock for the previous owner of the iPhone 6 because the previous owner is dead he passed on

This is a security feature, in part to deter re-selling of stolen devices. Click on the link to read more about 'Activation Lock for iPhone, iPad, and iPod touch' --> http://support.apple.com/HT201365

An activation lock can only be removed using information for the account that applied the lock. If you know this information follow the onscreen steps to enter the Apple ID and password used to set up the device, or enter the device passcode.

If the required information is not known, the official owner of the deceased person's things as can be proven by documentation needs to try one of these things:

Read this link to discover "How to request access to a deceased family member's Apple accounts" [and devices] - https://support.apple.com/HT208510

OR

- Apple can remove a lock if a person can provide sufficient evidence of valid ownership. Do this online here --> https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what constitutes proof. This might be similar to what is required in the previous section.

Visit How to request access to a deceased family member's Apple accounts - Apple Support.