pH 1 means $[\text{H}^+] = 0.1\ \text{M}$; pH 2 means $[\text{H}^+] = 0.01\ \text{M}$.

Using dilution formula $C_1V_1 = C_2V_2$:

$0.1 \times 1 = 0.01 \times V_2$

$V_2 = 10\ \text{L}$

Volume of water to add $= V_2 - V_1 = 10 - 1 = \mathbf{9.0\ \text{L}}$