Since you didn't mention what is the terminal you want to make the default one, I'm going to use
termite as an example for the desired terminal (replace it with the one you prefer).
First open the file
~/.profile and add this line
Then open your i3 config file and replace every instance of
i3-sensible-terminal (for more information on that script see
bindsym $mod+Return exec terminal
bindsym $mod+Return exec i3-sensible-terminal
Save your config file and logout/login. After that your default terminal should have changed to
termite (as an example).
If you want
dmenu to open terminal based applications in your desired terminal (
termite in this example), open the file
~/.dmenurc and change the entry
Hope that helps!