Swap every em dash for the punctuation you prefer, without leaving odd gaps around it.
Each em dash and the spaces around it are matched together and replaced by one clean piece of punctuation. You can choose a comma, a spaced hyphen, a plain hyphen, a colon, a period or a single space, or delete the dash completely.
The period option also capitalizes the next letter so the new sentence starts correctly. Turning on the en dash option catches numeric ranges and the smaller dash some editors insert, and the double hyphen option catches the typewriter style stand-in for a dash.
A comma is the safest for asides, since an em dash usually marks a pause. Use a colon when the dash introduces an explanation, and a period when each side of the dash could stand as its own sentence.
No. Single hyphens in words like well-known are never changed. Only the dash characters you select, and optionally double hyphens, are replaced.
The hyphen joins words, the en dash is about as wide as an n and marks ranges such as 10 to 20, and the em dash is as wide as an m and marks a break in a sentence. Some people call the em dash an em hyphen.