Use standard WWW browser - there's embedded WWW server inside of phone allowing you to configure the device. Follow Phone Administration Guide (beware, SPA525G is not the same model as SPA525G2).

Note there may be administrator password set on phone. Ask seller for it if set. If not known then reset phone to factory default. If not possible (reset to factory default may be disabled by configuration) then you are out of luck. Return device to seller.

Also, some phone's has been sold as OEM/restricted unit. You may not be allowed to change configuration of phone of such kind at all. Return device to seller in such case.

By the way, wrong forum for your's question. Better to ask in SPA IP Phones