Question: [Tuesday, 25th Iyar 5780]
I accidentally wore my Shel Yad of Rabbeinu Tam instead of Rashi and then noticed right away prior to putting on my Shel Rosh. What am I supposed to do? Should I take off the Shel Yad and wrap the Tefillin of Rashi with a blessing? Should I finish wrapping Rabbeinu Tam and then put on Rashi?
In such a case that you have already fastened your Shel Yad onto your arm I would suggest that you remove your Shel Yad of Rabbeinu Tam and put on your Rashi Tefillin without a blessing, without making any interval in-between.
Sources: See Minchas Elazar 1:25 in his understanding of Divrei Chaim 2:82 based on Kabbalah of Chozeh Melublin; Chaim Sheol 1; Ben Ish Chaiy Vayeira 24; Mikdash Me’at 34:7; Avnei Tzedek 1:2; Peri Hasadeh 4:53; Betzeil Hachachmah 4:81; Kinyan Torah 7:3; Mishnas Yosef 5:15; Tehsuvos Vehanhagos 3:10; Poskim in Miasef Lekol Hamachanos 34:5; The following Poskim rule that one is to always wear the Tefillin of Rabbeinu Tam prior to Rashi: Levush 34:2; 1st approach in Ateres Zekeinim 34; Har Tzevi 1:36; See here for the full details of this subject: https://shulchanaruchharav.com/halacha/chapter-4-tefillin-of-rabbeinu-tam/ by footnote 130