By reading the fine details of the question, carefully and analytically, I have determined that there's no list of modifications to choose from.
The strength of the magnetic field of a solenoid depends on the electric current in its coil windings, the number of wire turns in its coil windings, and the material in its core.
In order to <em>DE</em>crease the strength of its magnetic field, any one or more of these steps could do the job:
-- DEcrease the electric current in its coil windings. This can be accomplished by decreasing the voltage of the power source that energizes the coil, and/or increasing the resistance of the wire in the coil.
-- DEcrease the number of wire turns in the coil.
-- If the solenoid has anything in its core, change the core to something with a lower magnetic 'permeability'. An Iron core will produce the greatest magnetic field strength. Air, vacuum, or NO core will produce the lowest magnetic field strength.
