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, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
As long as those people have the intention to perform `Umrah as part of their journey, they should assume Ihram (ritual state into which a Muslim must enter to perform Hajj or 'Umrah) in the Miqat (a stated place where it becomes impermissible for a pilgrim or Mu`tamir to pass except in the state of Ihram) which they pass by or along, by air, land, or sea.
As long as they go to Madinah first, then Makkah, they have to perform Ihram for `Umrah from the Miqat of the people of Madinah: Dhul Hulayfah, which is currently known as Abyar `Ali. Whoever passes by Madinah intending to go to Makkah to perform the rituals of `Umrah, should perform Ihram from the Miqat of Madinah. This is according to what the Prophet, sallallahu 'alayhi wa sallam, said when he specified the Miqats: “So these spots are for these places and for people of other places who come through them with the intention of performing Hajj and `Umrah.”
Al-Kharashi stated in the explanation of Mukhtasar Khalil: “When a Najdi, Iraqi, Yemeni person or the residents of other countries pass by Al-Hulayfah, they should assume Ihram from it and should not pass it to go to another Miqat.” (End quote)
It is permissible for the Muhrim (person in a state of Ihram) to take a shower and take off or change the Ihram clothes, but he should beware of using any scented substance while taking a shower. Ibn Qudamah stated in Al-Mughni: “The Muhrim is allowed to wash his head and body gently, as `Umar and his son (Abdullah) did so and `Ali, Jabir, Sa`eed ibn Jubayr, Ash-Shafi`i, Abu Thawr, and As’hab ar-Ra’y (scholars advocating the use of reasoning to arrive at legal rulings “Hanafis") permitted doing so.”
It is reported in Sahih Al-Bukhari that Abu Ayyub Al-Ansari washed his head while being in a state of Ihram. He said: “This is how I saw him (the Prophet, sallallahu 'alayhi wa sallam) doing.”
Allah knows best.