bring up the client with the new, improved  custom client letter.

in that little window... hit 'EDIT"

the letter comes up.

Right click,drag, and highlight  the entire letter. when you do that... across the top toolbar... to the immediate right of the scissors is the copy button (looks like 2 pages of paper). hit that button.

then just exit that client specific letter.

then top toolbar>>tools>>letters>>manage standard letters.  Click on Standard client letter>>Edit.

brings that letter up.  Then either at the very top of that letter or the very bottom of that standard letter, put your cursor, and hit the Paste Button, which is immediately to the right of the copy button you used above.  now...just highlight the old standard letter part...hit delete...and you have your revised letter as the Standard Client Letter.  be sure and save before exiting

i do this first thing each year...basically copying my "standard letter" from the prior year using the copy and paste buttons in the letter editor