Replace mode is similar to insert mode, except typing characters replaces any existing characters, rather than inserting them. When typing reaches the end of a line, replace mode simply inserts the new characters.
Let's take a look at how this works. Starting from the following buffer:
Initial Conditions
FloralWhite
Orange
Wheat
Blue
DeepSkyBlue
LightSkyBlue
LavenderBlush
DarkRed
DarkGray
NORMAL20%2:1
we start by hitting R to enter replace mode.
Enter REPLACE ModeR
FloralWhite
Orange
Wheat
Blue
DeepSkyBlue
LightSkyBlue
LavenderBlush
DarkRed
DarkGray
REPLACE20%2:1
Now that we are in replace mode, we can simply type new keys to replace the existing content. Note that as we continue typing past the existing text, new text is inserted into the buffer.
Replace Text In Bufferthis is some new text
FloralWhite
this·is·some·new·text
Wheat
Blue
DeepSkyBlue
LightSkyBlue
LavenderBlush
DarkRed
DarkGray
REPLACE20%2:22
Now that we are done replacing text, let's hit Esc to return to normal mode.
Return to NORMAL mode<Esc>
FloralWhite
this·is·some·new·text
Wheat
Blue
DeepSkyBlue
LightSkyBlue
LavenderBlush
DarkRed
DarkGray
NORMAL20%2:21
That's about it. You can find more details about replace mode in the Neovim docs