accidental nitrogen tank instead of helium

The simplest is to loosen the Swagelok fitting on the back of the GC at the injection port Electronic Pressure Control Module.  If you can hear it hissing it is greater than about 1 L/min.  How long to purge depends on how far away the tank is located and how many/what type of traps are inline.   Remember to tighten it back up again.  Swagelok fittings do not need to be super tight to seal, and overtightened is often much worse than sufficiently tightened.

If you have a GasClean or Big Universal Trap or both on the line, the nitrogen is in the packing material. The best way to purge those out is a bit more complicated. See slide 5: (+) How to Change Your Helium GCMS Carrier Gas Tank - Two ways - Files - GC/MS - Agilent Community

Why not nitrogen?

You would think that using nitrogen as carrier gas would be a great idea.  Nitrogen is readily available, inexpensive, and relatively inert.   Nitrogen’s ionization characteristics result in approximately nine times more charge than helium.  This causes a very high ion‐source space‐charge – too many ions too close together repel each other.  Using nitrogen is about like using 10.8 ml/min of helium instead of the recommended 1.2 ml/min.  It reduces the GC‐MS EI sensitivity by about 20 times!  <this is why we must get the nitrogen down after changing gas filters>