The pH of a solution is 6. Its $\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]$is decreased by 1000 times. Its pH will be

$\mathrm{PH}=6={10}^{-6}\left[{\mathrm{M}}_1\right]$

$\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]=decreased by 1000 = {10}^{-3}$

${\mathrm{M}}_2={\mathrm{M}}_1\times \mathrm{x}={10}^{-6}\times {10}^{-3}={10}^{-9}\mathrm{PH}=9$

Here, also pH of acid, acid cannot become a base but as it is a weak acid it is near 6.96.