Not sure what you mean by you want it changed for each letter, but you don't want to save it. I assume you mean that you want to edit some data in every letter that is being sent, until you change that value. And you don't mind that the previous edit wasn't saved.
You could create a global field and let the user enter the data there. Then merge that data into the text the same way as the other merge fields. Obviously it will always appear in the same position in the standing text.
Sorry I know I didn't explain myself well. What I meant was like if a letter needs to edit him/her depending on recipient or specific words/info like that, the rest of the letter being standardized.
And I'm new to this design stuff, what do you mean by create a global field? Sorry for my ignorance
You may find this demo file helpful: http://www.4shared.com/file/7DdcpW11/MessageTemplateBuilder.html
Apologies in advance for the fact that a recent policy change on the part of 4shared now requires you to sign up before you can download, but at least doing so does not cost you any $$$ at this time... (And I'm going to start looking for a replacement site to use in place of 4shared later today...)
The ability to edit data in a FileMaker database requires defining a field for that purpose--can't set up any kind of "text box" just for tempoarily editing data.
To get a Global Field as Sorbsbuster describes, requires locating the field in Manage | Database | Fields and double clicking it to open field options. Then you click the storage tab to see a check box where you can select "global" storage.
If the changes are all of the type 'him' 'her', 'she', 'he', then you can create a standard text box with placeholders that you can edit, eg:
"I saw <<MergeField FirstName>> on <<VisitDate>> and |THEY| showed me |THEIR| notes. [blah blah...]" can be them automatically personalised.
The field that is used in the letter uses the substitute function to replace the instances of |THEY| with a calculation based on their gender, eg:
Gender = "Female" ;
Substitute (MyMailMergeText ; "|THEY| ; "she" ) ;
Substitute (MyMailMergeText ; "|THEY| ; "he" )
(There is a neater way to write that but I think that is easier to see the logic of.)