1to change one's tune
2to modify one's previous remark
3to change the way one addresses sb (as when one marries and starts to call one's husband's parents 爸爸 and 妈妈|妈妈)
Example
nǐ你bù不gǎi kǒu改口de huà的话，wǒ我míng tiān明天jiù就lái来děng等nǐ你le了。
你不改口的话，我明天就来等你了。
I'll expect you tomorrow unless you let me know otherwise.