KUWAIT (AP) _ Kuwait's defense minister, Sheik Salem al-Sabah, swapped jobs Tuesday with Interior Minister Sheik Nawaf al-Ahmed in a Cabinet reshuffle.

A decree from the emir, Sheik Jaber al-Ahmed al-Sabah, announced the changes and said they were designed to ''cope with changing circumstances,'' according to state-run Kuwait radio.

The reshuffle affected 10 of the 22 Cabinet ministers, with three ministers resigning.

The defense minister, a cousin of the emir, held the post for eight years. In the past year Iran has fired at least seven Chinese-made Silkworm missiles at Kuwait, which lies closest to the Iraq-Iran warfront.

The interior minister, a brother of the emir, has confronted sabotage believed masterminded by the Iranians, including an attempt on the life of the ruler in May 1985. He also had held his portfolio for eight years.

Iran accuses Kuwait of aiding Iraq and has been threatening Kuwaiti shipping lanes in retaliation for Iraqi attacks on its own oil tanker routes. The Iran-Iraq war now is in its eighth year.

The emir accepted the resignations of Planning Minister Mohammed Suleiman Sayed Ali, Communications Minister Khalid al-Jumaian, and Minister of State for Municipal Affairs Abdul-Rahman al-Ghunaim. Al-Jumaian and al-Ghunaim were earlier elected members of the assembly.

Three new ministers entered the Cabinet. They were Abdul-Razzak Yousef al- Abdul Razzak, health minister; Hmoud al-Rquba, minister of electricity and water, and Abdullah al-Sharhan, minister of communications.

The outgoing Cabinet was formed in the summer of 1986.