how to deauthorize my apple a/c from all computer which i no longer need

Check iTunes -> Account -> View Account.

Once authenticated there's an option to deauthorize all devices.

Deauthorize your computer using iTunes - Apple Support

I'm trying to back up my iPhone on my new laptop as my old one have crashed. I am told I have exceeded 5 devices and need to deauthorize a device.

I am unable to do it on the old devices as I no longer have access to them. I tried logging into apple --> clicked on Store --> login --> click on my name, however it does not give me options to remove devices. I clicked on view account but there was nothing useful there.

I tried logging on itune on my iphone, however it only give me option to remove that device.

Please help, I'm hoping to update my phone prior to going overseas for a few week.

Thanks.