With rise in temperature, density of a given body changes according to one of the following relations 

Since, with increase in temperature, volume of given body increases, while mass remains constant so that density will decrease.

$\begin{array}{l}\text{ i.e. }\frac{\mathrm{\rho }}{{\mathrm{\rho }}_0}=\frac{\mathrm{m}/\mathrm{V}}{\mathrm{m}/{\mathrm{V}}_0}=\frac{{\mathrm{V}}_0}{\mathrm{V}}=\frac{{\mathrm{V}}_0}{{\mathrm{V}}_0(1+\gamma \mathrm{\Delta \theta })}=(1-\mathrm{\gamma \Delta \theta })\\ \therefore \mathrm{\rho }={\mathrm{\rho }}_0(1-\mathrm{\gamma \Delta \theta })\end{array}$