Question: I was travelling and due to losing a few hours, didn’t manage to daven shacharis on time. What should I do now?
Answer: The Shulchan Aruch (OC 89:1) writes that one should daven shacharis within four hours from alos hashachar (dawn). The Rema writes, however, that one may daven shacharis until chatzos (midday) if necessary.
The Gemara (Berachos 26a) writes that one who accidentally missed shacharis, mincha or maariv can make it up by repeating the amida in the following davening. This is known as tashlumin. The Shulchan Aruch (OC 108:7) and Mishna Berura (108:1; 22) stress that one who deliberately missed a tefilla does not have this opportunity.
The Shulchan Aruch (OC 108:1) writes that one must ensure to say the regular amida first followed by the tashlumin (See Aruch Hashulchan OC 108:9).
As this tashlumin should be said soon after the regular amida, the Mishna Berura (108:11) writes that one mustn’t interrupt even to learn something, though one doing tashlumin for shacharis should listen to chazaras hashatz and say tachanun and ashrei first (See Rivevos Ephraim 1:170; 3:142; 8:37).
R’ Avraham Yeshaya Pfoifer (Ishei Yisrael 30:n5) writes that as each of these amidos should be the same, one (who davens nusach ashkenaz) says shalom rav instead of sim shalom even when repeating the amida.
While we no longer wear tefillin while davening mincha (See Igros Moshe OC 4:34), R’ Pfoifer (Ishei Yisrael 19:n46) writes that one who hasn’t yet worn them should do so then.
In conclusion, if one didn’t manage to daven shacharis, one should daven mincha as normal, though repeat the amida again. They should listen to chazaras hashatz (if in shul) first, and say both tachanun (when relevant) and ashrei before the tashlumin.