What's the terminal command for auto pressing [Y]?

for ex: if I do like

sudo apt-get upgrade

Then I might get the y/n question before installing an update. What's the parameter to auto accept it.

There are two ways to do this:

Apt-get specific:

$man apt-get
[...]
-y, --yes, --assume-yes
       Automatic yes to prompts; assume "yes" as answer to all prompts
       and run non-interactively. If an undesirable situation,
       such as changing a held package, trying to install a unauthenticated
       package or removing an essential package occurs then apt-get will abort. 
[...]
--force-yes
       Force yes; this is a dangerous option that will cause apt to continue
       without prompting if it is doing something potentially harmful.
       It should not be used except in very special situations.
       Using force-yes can potentially destroy your system!

General: the yes command:

$man yes
yes - output a string repeatedly until killed [...]
DESCRIPTION
   Repeatedly output a line with all specified STRING(s), or 'y'.

Since yes can't know anything about apt-get internals, it is probably equivalent to --force-yes and thus dangerous. If you still want to do it, you pipe the output of yes into apt-get:

yes | apt-get update