The upper room was actually an upstairs room of a house at the time of the last supper. It is the place where the Apostles stayed in Jerusalem. Since the disciples continued to have access to this room after Jesus' death, burial, and resurrection it is possible that the home belonged to one of Jesus' disciples but this is not specified. (Some speculate this was Mark's home.)
The events that occurred in the "upper room," also known as the Cenacle," are described in Matthew 26:1-29, Mark 14:12-15, Luke 22:7-20, and John 13:1-38. During these last hours that Jesus spent with his beloved friends He ate with them, instituted the New Covenant in His blood, gave them last-minute instructions and encouragement, and prayed over them.
Prior to the last supper Jesus displayed His humility and his servanthood when he rose from the table and began to wash the feet of His disciples (John 13:4).
It was during the last supper that Jesus identified Judas as the traitor who would betray Him to authorities and bring about His arrest. He instructed Judas to leave and do what he had to do.