To replace a word, just select it from the pop-up menu.

A list of possible corrections appears at the top of the contextual pop-up. To correct a flagged word that you recognize is wrong, control-click (right-click) the word. Make a habit of looking back at material you've already typed to see whether the underline appears. You may sometimes encounter a slight delay between typing words and seeing them get flagged. When enabled, the spell checker marks words that it does not recognize with a dashed red underline. If it is not checked, just select it to check it. Choose Spelling from the contextual pop-up menu and then make sure that Check Spelling as You Type is checked. To enable spell checking in an application, control-click (or right-click on a two-button mouse) a text field or text entry area. Many OS X applications including Mail, Pages, TextEdit, Safari and iChat AV (among others) allow you to take advantage of this feature to find and correct spelling mistakes. Mac OS X offers a system-wide built-in spell checker.

How to enable right click on mac