The short answer is:
You don't.
dash is a direct descendent of the Almquist Shell (ash). ash never featured line editing support, and neither does dash. Busybox ash does, so if you must an ash variant for some reason and have line editing, etc., use Busybox ash. Nobody's going to bother with adding readline support, since dash's primary use is for running shell scripts.
Unless, of course, you're willing to code support for readline in dash, and maintain such a patch yourself...
The long answer is:
Compile with libedit
If you look at dash's manpage:
-V vi Enable the built-in vi(1) command line editor
(disables -E if it has been set).
-E emacs Enable the built-in emacs(1) command line editor
(disables -V if it has been set).
These only work if dash was compiled with --with-libedit. It isn't, neither in Ubuntu, nor, apparently, in Debian.
You can build it thus:
git clone https://git.kernel.org/pub/scm/utils/dash/dash.git
cd dash
./autogen.sh
./configure --with-libedit
make
Then run:
src/dash -E
You should be able to use the arrow keys to edit the current command.