Start from the definitions of specific humidity s and relative humidity r:

s = density of water vapour / density of moist air = (R_A/R_W) * (e/(p + (R_A/R_W -1)))

r = e /e_s(T)

where 

e = water vapour pressure

e_s(T) = water vapour saturation pressure, which is a function of temperature T

p = air pressure

R_A = gas constant of air

R_W = gas constant of water vapour

Putting both equations together will give the relation between specific humidity and relative humidity. Assuming p >> e, one can approximate

s = 0.622 e/p, which leads to 

r = s p / (0.622 e_s(T))

You may want to check my posts at the signalled discussion ― addressing calculation of relative humidity and humidity ratio for humidified hydrogen gas: https://www.researchgate.net/post/In_a_100_humidified_hydrogen_what_would_be_the_mass_of_water_molecules_per_unit_mass_of_hydrogen

thank you so much Peter Schluessel and Carlos Araújo Queiroz

I have some limited parameters (temperature(max and min), precipitation, specific (relative) humidity) that download from website "www.earthsystemgrid.org" in format "netcdf" until 2100. Unfortunately parameters of this website is very limited, so on i want to convert specific (relative) humidity to maximum or minimum relative humidity. please guide me How do this? Thanks and regard.

Specific humidity : density of water vapour / density of moist air (unit= g/kg)

The website (https://earthscience.stackexchange.com/questions/2360/how-do-i-convert-specific-humidity-to-relative-humidity) would be helpful.

Yan Tong Thank you.