Why did Jesus change the sabbath day from 1st to 2nd?

He didn't.

Firstly, you've got the numbers wrong: the Jews celebrate the Sabbath day on the seventh day of the week, which is Saturday.

Christians instead meet on Sunday, which is the first day of the week.

Jesus didn't change the "Sabbath", his followers did... and not immediately; the ones among them who had been Jews kept observing the Sabbath, but they also met on "the Lord's day", Sunday, because that was the day on which Jesus rose from the dead. It wasn't until there were a substantial number of non-Jewish Christians (who had no tradition of observing the Sabbath at all) that Sunday became predominant.