If you have taken the weight of the tyres, I assume with axle stands or similar, then as the tyres are no longer in conatct with the ground you can reduce the pressures accordingly. However, I doubt it will make much difference to tyre life. Whatever you do, the clock is ticking and once they have reached 5 years from the date of manufacture, you are really wise to renew them.
If you can, remove the wheels and store them in a dark place, if not try covering them so light does not reach them so much. If however, you have only partially taken weight of the tyres, then keep the mnormal pressures and rotate the wheels monthly, if you can.