1: In the main document, press ALT+F9 to display field codes.

Then immediately following the name of the field (ADDRESSBLOCK, GREETINGLINE, or MERGEFIELD), type a space, and then type one of the following formatting switches:

\* Caps

Capitalizes the first letter of each word.

\* FirstCap

Capitalizes the first letter of the first word.

\* Upper

Capitalizes all letters.

\* Lower

Formats all letters as lowercase.

Then  Press ALT+F9 again to hide field codes.