Add more solvent ?

For the calculations, if Ci and Cf are the initial and final concentrations, Vi and Vf the volume to be taken adn final solution volume, you have

Vi x Ci = Vf x Cf

And, therefore: Vi = (Vf x Cf)/Ci

PS: Only works on well-disperse solution (no precipitate)

Ethanol can be added to reduce concentration. As suspension is diluted by added mass of ethanol (Me), suspension's mass increases from Mi to Mf, being Mi + Me = Mf. For the suspended solid, mass balance reads: Ri·Mi = Rf·Mf, being Ri and Rf its initial and final mass concentrations (mass of solid : mass of suspension). As we eliminate Mf between last equations, one obtains: Me = [(Ri/Rf) -1]·Mi. By considering ethanol density (ρe) and initial density of suspension (ρi), the volume of ethanol to be added to initial volume of suspension (Vi) will be given by: Ve  = (ρi/ρe)·[(Ri/Rf) -1]·Vi. An alternative simpler but approximate expression, Vi = (Rf/Ri)·Vf, can be obtained from the above written mass balance for the suspended solid, while accepting that initial density of suspension closely matches final density.

The suspension is stabilized by pH adjustment and also by adding ethanol we can able to reduce the concentration.....

For a recent but somewhat related question, also dealing with the concentration of a suspension, you may check: https://www.researchgate.net/post/Smart_way_to_calculate_volume_fraction_of_a_solid_ferrofluid_in_a_solvent_or_solids_fraction