Write Umlaut in Computer 2

Linux keyboards (Ubuntu example):

In Ubuntu Linux (or I suppose any type of Linux), first use Keyboard preferences to designate a Compose key, for example one of the <Ctrl> keys.

Then you can type:

For lowercase "ü" press <Ctrl> " u   (Control - double quote -u)
For uppercase "Ü" press <Ctrl> " U

Mac OS:

Hold down the <Option> key and press "u". Then release <Option> and type "u" or "U". (This also works to put those two dots above other vowels as well.)

Chrome OS:

On the "US extended" keyboard you'll find lowercase "ü" on the "y" key, and you can get the uppercase "Ü" by typing an uppercase "Y". See the online Chrome help for info on setting up the extended keyboard.
On the standard keyboard, you can also enter them in Unicode hex:

For lowercase "ü" press <Ctrl> <Shift> u, then type "fc" (without quotes) and <space bar>
For uppercase "Ü" press <Ctrl> <Shift> u, then type "dc" (without quotes) and <space bar>

HTML

The html is &uuml; for "ü", and &Uuml; for "Ü".