You can use the ROL Rotate bits left (or ROR, rotate bits right) to shift the low word to the high word, and vice versa. Rotating a UDINT 16 bits will swap the words in order.
In Structured Text:
newUDINT := ROL(oldUDINT, 16);
See the AS Help about ROL (or ROR) for additional information:
B&R Online Help (br-automation.com)