Open YouTube and sign in to your account
Go to your profile picture
Select Your data in YouTube
Scroll to YouTube History
Click Manage your YouTube History
Choose Delete
Select Delete all time
Confirm the deletion
To delete watch history only, go to History
Click Clear all watch history
Confirm the action
To delete search history only, go to History
Click Manage all history
Select Delete
Choose Delete all time
Confirm the deletion
Turn off YouTube History if you do not want future history saved
Repeat the same steps on each device if needed