In FF
open the font you want to edit. Make sure the font’s copyright allows you to edit it. 



In FF
delete the old letter:
Using the arrow tool inside the letter viewer, select all the nodes in the letter, or press ctrl-a to select all. press the Delete key on the keyboard (Backspace will not do).
replace it with the new:
in the letter viewer still, File>import>import your new .svg that you just edited. 

For this letter, and probably all letters that you change you will probably need to also do the following fixes:
Select all the nodes in the letter. Go to 
Elements>Round>Integer
and 
Elements>Correct Direction

When all your letters are completed go to the main font window and Build Font. Select TrueType. All other defaults should be fine.