Are the instructions given in the “Planar Binding” spell part of the casting time, or can instructions be amended during the spell duration?

The Planar Binding spell is a 5th level abjuration with verbal, somatic and material components and a casting time of 1 hour that allows the target a Charisma saving throw. If the target fails the save, it “must follow your instructions to the best of its ability”.

Are the instructions specified as part of casting the spell? Or are the instructions given after the spell has finished casting? Can instructions be updated/amended throughout the duration of the spell, or is there a one-time period during which the instructions must be fully specified?

Although the duration for the 5th level version of this spell is 24 hours, the duration increases as higher spell slots are used, lasting a year plus a day when a 9th level spell slot is used. I mention this because the longer the spell lasts, the more important the answer to this question becomes.