  The X-ray pulsar RX J0440.9+4431 went through a giant outburst in 2022 and
reached a record-high flux of 2.3 Crab, as observed by Swift/BAT. We study the
evolution of different spectral and timing properties of the source using NICER
observations. The pulse period is found to decrease from 208 s to 205 s, and
the pulse profile evolves significantly with energy and luminosity. The
hardness ratio and hardness intensity diagram (HID) show remarkable evolution
during the outburst. The HID turns towards the diagonal branch from the
horizontal branch above a transition (critical) luminosity, suggesting the
presence of two accretion modes. Each NICER spectrum can be described using a
cutoff power law with a blackbody component and a Gaussian at 6.4 keV. At
higher luminosities, an additional Gaussian at 6.67 keV is used. The observed
photon index shows negative and positive correlations with X-ray flux below and
above the critical luminosity, respectively. The evolution of spectral and
timing parameters suggests a possible change in the emission mechanism and
beaming pattern of the pulsar depending on the spectral transition to sub- and
super-critical accretion regimes. Based on the critical luminosity, the
magnetic field of the neutron star can be estimated in the order of 10$^{12}$
or 10$^{13}$ G, assuming different theoretical models. Moreover, the observed
iron emission line evolves from a narrow to a broad feature with luminosity.
Two emission lines originating from neutral and highly ionized Fe atoms were
evident in the spectra around 6.4 keV and 6.67 keV (higher luminosities).
