How can i delete my back up to my old iphone..? So i can started back up to my new device

You can keep backups of both, but if you want to delete, see Manage your iCloud storage - Apple Support

I have a similar problem. However for me I wanted to select the 6s backup for my6s when different options are given for back up However by mistake I clicked on 5s backup. Now I want to delete this 5s backup and select the 6s. Will this step help me too?

Go to Settings > iCloud > Storage > Manage Storage

Tap the device and click "Delete backup".

My old backups are on my Mac and I can't find how to delete. I have a defunct apple id in several which is problematic.

hi mcintoshjr,

for this issue you would go to your preferences in iTunes > Devices and select what back up you'd like to delete.

all greyed out, no option