有则改之，无则加勉(《论语·学而》)

correct mistakes if you have made any and guard against them if you have not (The Analects of Confucius)
知无不言，言无不尽；言者无罪，闻者足戒；～。(苏轼《策略第三》)Say all you know and say it without reserve;blame not the speaker but be warned by his words;correct mistakes if you’ve made any and guard against them if you’ve not.