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Ultraviolet (UV; rest-frame $\sim1200-2000$ A) spectra provide a wealth ofdiagnostics to characterize fundamental galaxy properties, such as theirchemical enrichment, the nature of their stellar populations, and their amountof Lyman-continuum (LyC) radiation. In this work, we leverage publicly releasedJWST data to construct the rest-frame UV-to-optical composite spectrum of asample of 63 galaxies at $5.6<z<9$, spanning the wavelength range from 1500 to5200 A. Based on the composite spectrum, we derive an average dust attenuation$E(B-V)_\mathrm{gas}=0.16^{+0.10}_{-0.11}$ from \hb/\hg, electron density $n_e= 570^{+510}_{-290}$ cm$^{-3}$ from the [O II] doublet ratio, electrontemperature $T_e = 17000^{+1500}_{-1500}$ K from the [O III] $\lambda4363$/ [OIII] $\lambda5007$ ratio, and an ionization parameter$\log(U)=-2.18^{+0.03}_{-0.03}$ from the [O III]/[O II] ratio. Using a direct$T_e$ method, we calculate an oxygen abundance$12+\log\mathrm{(O/H)}=7.67\pm0.08$ and the carbon-to-oxygen (C/O) abundanceratio $\log\mathrm{(C/O)}=-0.87^{+0.13}_{-0.10}$. This C/O ratio is smallerthan compared to $z=0$ and $z=2$ - 4 star-forming galaxies, albeit withmoderate significance. This indicates the reionization-era galaxies might beundergoing a rapid build-up of stellar mass with high specific star-formationrates. A UV diagnostic based on the ratios of C III]$\lambda\lambda1907,1909$/He II $\lambda1640$ versus O III] $\lambda1666$/He II$\lambda1640$ suggests that the star formation is the dominant source ofionization, similar to the local extreme dwarf galaxies and $z\sim2$ - 4 HeII-detected galaxies. The [O III]/[O II] and C IV/C III] ratios of thecomposite spectrum are marginally larger than the criteria used to selectgalaxies as LyC leakers, suggesting that some of the galaxies in our sample arestrong contributors to the reionizing radiation.

Characterizing the Average Interstellar Medium Conditions of Galaxies at $z\sim$ 5.6-9 with UV and Optical Nebular Lines