To put it bluntly, you're going to have to improve. 恕我直言,你还得改进。
He was annoyed, to put it mildly (= he was very annoyed). 说得好听一点,他有点儿不太高兴。
To put it simply, the business must change or it will go under.
She found her new colleagues uninspiring, to put it bluntly.
They are hardly the most popular couple in town right now, to put it mildly.
See also: put (EXPRESS)