Subchannel data updating of high-order modulation format signals using cascaded sum- and difference-frequency generation in a single periodically poled lithium niobate waveguide is demonstrated. One quadrature phase-shift-keying subchannel of a 16-quadrature amplitude modulation (QAM) signal at $40\mathrm{Gbit}/\mathrm{s}$ is successfully updated, with an optical signal-to-noise ratio (OSNR) penalty of $\sim 2\mathrm{dB}$ for return-to-zero and $\sim 4\mathrm{dB}$ for non-return-to-zero at a bit-error rate (BER) of $2\times {10}^{-3}$. Simultaneous processing of four wavelength-multiplexed 16-QAM signals with an average OSNR penalty of 4.5 dB at a BER of $2\times {10}^{-3}$ is also demonstrated.