There are two ways to undo in Texifier.
Shake to undo: the standard iOS shake gesture will open the following

Undo button: The keyboard has an undo button on the numbers pane, and a redo button on the symbols pane.

There are two ways to undo in Texifier.
Shake to undo: the standard iOS shake gesture will open the following
Undo button: The keyboard has an undo button on the numbers pane, and a redo button on the symbols pane.