有则改之，无则加勉

yǒu zé gǎi zhī , wú zé jiā miǎn

correct any mistakes you made, but maintain your good record if you did not make them (idiom)

有

yǒu

to have; there is

则

zé

(literary) (conjunction used to express contrast with a previous clause) but; then

改

gǎi

to change

之

zhī

(possessive particle, literary equivalent of 的[de5])

，

无

wú

not to have

则

zé

(literary) (conjunction used to express contrast with a previous clause) but; then

加

jiā

to add

勉

miǎn

to exhort