Angels have the Phone Box
As bobkn said, you can't just "copy" an ISO image to a DVD - it has to be "created". If you have an ISO of, say W10 Home x64, then that will fit on a single DVD, and you can follow dougout's instructions to right-click>burn DVD. However, if you've created an ISO which would install both W10 Home and Pro, in both 32 and 64 bit, then that won't work, because it would be too big for a regular DVD and you'd need a dual-layer DVD, of which the native Burn can't handle. In that case, you either make a new ISO with only the version/architecture you need, or use 3rd-party burning software which can handle dual layer.
The ISO itself can be save to an external hard drive, for creating the DVD at some later time. The ISO can also be mounted from within the W8.1 OS, to do an upgrade that way.