Zhùshǒu biānjìng de bùduì měi sān gè yuè huànfáng yí cì.
Troops stationed at the border rotate out every three months.
Tips
usage
换防 is the standard military term for one unit replacing another at a defensive position, think of British 'changing of the guard' but in operational rather than ceremonial contexts. Don't confuse with 防御 (defense) or 防守 (to defend).
