Q. If ones shirt became stained, is it correct for a Ben Torah to immediately change it despite the Bittul Torah or better wait until after Seder or other convenient time.

A. Horav Aharon Miller Shlit’a recommended just to cover if possible the area of the stain with ones jacket or coat and to continue learning.
Horav Shlomo Miller’s Shlit’a opinion is that if it cannot be done, one shold continue learning Torah as is,

See also questions 758, 2282, 1991, 2768, and 3043.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a