If your flight plan is A, B, C, D, E, F
your are already at B and ATC ask you to resume at C, you simply press C LSK on the LEGS page, then press LSK1 (active waypoint) to override active waypoint with C and shift the entire flight plan up, then EXEC then engage LNAV, you may also engage VNAV if cleared to resume vertical profile as well.
The aircraft will fly to C and resume.
Hope this helps
edit: i am unfamiliar about p2ATC, but on real life and also over online networks like Vatsim or Pilotedge. Is quite common depending on traffic conditions, you are flying at B and ATC tells you cleared direct to E. Procedure is the same:
-Press E LSK on the legs page, press LSK1 to override active waypoint, press EXEC and now you are heading direct to E skipping everything in between and the FMC will resume the flight plan from there