To overwrite characters in filenames:
Drag the files you want to rename from the Finder to the file table.
Click
and choose Characters.
Click Overwrite.
Type the characters you want to insert into the “Replacement” box.
Enter the character position at which to insert the text in the “At position” box.
Click Apply.
. In this case a character position of 0 means the text is overwritten after the last character, 1 before the last character, and so on. This is useful if each filename has a different length and you want to, say, overwrite text before the second-to-last character (character position 2).