This paper presents updated estimates of source parameters for GW150914, a binary black-holecoalescence event detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) in2015 [Abbott et al. Phys. Rev. Lett. 116, 061102 (2016).]. Abbott et al. [Phys. Rev. Lett. 116, 241102(2016).] presented parameter estimation of the source using a 13-dimensional, phenomenologicalprecessing-spin model (precessing IMRPhenom) and an 11-dimensional nonprecessing effective-onebody(EOB) model calibrated to numerical-relativity simulations, which forces spin alignment(nonprecessing EOBNR). Here, we present new results that include a 15-dimensional precessingspinwaveform model (precessing EOBNR) developed within the EOB formalism. We find goodagreement with the parameters estimated previously [Abbott et al. Phys. Rev. Lett. 116, 241102(2016).], and we quote updated component masses of 35+5−3 M⊙ and 30+3−4 M⊙ (where errors correspondto 90% symmetric credible intervals). We also present slightly tighter constraints on the dimensionlessspin magnitudes of the two black holes, with a primary spin estimate < 0.65 and a secondary spinestimate < 0.75 at 90% probability. Abbott et al. [Phys. Rev. Lett. 116, 241102 (2016).] estimated thesystematic parameter-extraction errors due to waveform-model uncertainty by combining the posteriorprobability densities of precessing IMRPhenom and nonprecessing EOBNR. Here, we find that the twoprecessing-spin models are in closer agreement, suggesting that these systematic errors are smallerthan previously quoted.