Find and Replace text in SQL Server Management Studio

To open the Quick Find dialog box press CTRL+F:

quick_find_ssms

To find the next match click 'Find Next' or press F3.

To open the Quick Replace dialog box press CTRL+H:

quick_replace_ssms

To find the next match click 'Find Next' or press F3.
There is an option to replace the current or all matched keywords.

For both Quick Find and Quick Replace dialog box, you can expand the 'Find Options' section to select some of the additional options:

quick_find_ssms_options

You can also select the location to look for the text specified in 'Find what':

quick_find_ssms_look_in

There is a feature in SQL Server Management Studio called Incremental Search that can be started by pressing CTRL + I from your Query Window.
The cursor icon will change to binoculars with an arrow indicating the search direction:

incremental_search_icon

...you can start typing the keyword you are looking for and cycle through the matches by pressing F3.