Assalaamu alaykum. I know that there are hadiths that say that Allah decends to the lower heaven in the last third of the night. However, in one hadith in Saheeh Muslim, it says that Allah decends after passing one third of the night. How can I reconcile between them?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

Both narrations of the hadeeth are authentic. Some scholars favored one narration over the other and considered the following narration as preponderant: "Our Lord, the Blessed and Exalted, descends every night to the nearest heaven when one third of the night is left, and He says, 'Who supplicates Me so that I may answer him? Who asks Me so that I may give to him? Who asks Me forgiveness so that I may forgive him?'", as this narration is reported by both Al-Bukhaari and Muslim, and for other reasons.

Some other scholars tried to reconcile between this narration reported in Saheeh Al-Bukhaari and Saheeh Muslim and the one reported in Saheeh Muslim which reads, "Allah descends every night to the nearest heaven when one third of the night has passed and says, 'I am the Sovereign, I am the Sovereign. Who is there to supplicate Me so that I answer him? Who is there to beg of Me so that I grant him? Who is there to beg forgiveness from Me so that I forgive him?' He continues like this till the day breaks." So they said, "We can reconcile between these two narrations without being unfair by saying that the descent of Allah varies."

Ibn Taymiyyah said, "If the Prophet also mentioned the descent after one third of the night has passed and after half of the night has passed, then his saying is the truth and he is truthful in everything he says. So the descent is of three types: The first is when one third of the night is over, then when half of the night is over, which is more preponderant, and finally, when one third of the night is left, and this is the most preponderant among the three types."

Allah knows best.