Air carrier First Officer reported GPS jamming in international airspace resulting in loss of navigation accuracy in oceanic airspace. The navigational accuracy improved upon exiting the ocean and after coast in.

Narrative

The on duty crew experienced GPS jamming/spoofing while I was on break. After exiting the affected airspace; both GPS receivers were still degraded and GPS updating was never reestablished. All of this is pretty typical for this flight. When my break ended; the Captain and flying FO briefed us on the situation and went on their break. After coasting out; we received a VERIFY POSITION message in the FMC scratchpad which made sense based on the increasing ANP value. Initially this message was acknowledged and cleared but after some browsing; found a FMC Position Update checklist in the unannunciated checklists and executed it. Step X of the checklist says to 'Select the NOW key for the most appropriate source.' The only option available at this step was 'inertial' at 2R which was selected because I thought surely the inertial position was more accurate than the FMC; and; incorrectly; I assumed this step was reversible. After selecting 'NOW'; the ANP immediately jumped from 4.0 to >16 and we were unable to reset back to the original FMC position. We informed dispatch and all appropriate ATC agencies that we were no longer RNP 10.0 capable and the flight continued without incident and acceptable ANP values were reestablished after coast in.