Emacs: Evaluating expressions

Use M-: to evaluate a single elisp expression. You can use this for example for setting variables. For example: If you enabled the mode auto-fill (M-x auto-fill-mode), the default wrap value for your lines is set to 70.

You can then use M-: to reset this to a more sane value by entering

(setq fill-column)

into the minibuffer.