Don't judge by appearances (勿以貌取人)

Don't judge by appearances
Don't judge a book by its cover

to get by...

to get by means "to have just enough of something such as money or knowledge so that you can do what you need to do.

Eg. My math isn't very good, but I get by. / I can't get by on such a small income.

Let sleeping dogs lie

"Let sleeping dogs lie "means to not talk about things which have caused problems in the past, or to not try to change a situation because you might cause problems.

Eg. His parents never referred to the shoplifting incident again. I suppose they thought it best to let sleeping dogs lie.