bigger changes

\begin{tikzpicture}[
  node distance = 8mm and 16mm,
  start chain = A going below,
  base/.style = {draw, minimum width=32mm, minimum height=8mm,
  	align=center, on chain=A,join},
  startstop/.style = {ultra thick,double,base, rectangle, rounded corners, fill=white},
  process/.style = {ultra thick,base, rectangle, fill=white},
  io/.style = {ultra thick,base, trapezium,
  	trapezium left angle=70, trapezium right angle=110,
  	fill=white},
  decision/.style = {ultra thick,base, diamond, fill=white},
  every edge quotes/.style = {auto=right},
  every join/.style=-Stealth]
  ]
  \node [startstop]       {Έναρξη};     % <-- A-1
  \node [io]              {Εισαγωγή}; %A-2
  \node [decision]        {Ερώτηση}; %Α-3
  \node [io, right=of A-3]              {Απάντηση}; %A-4
  \node [startstop]       {Τέλος};      % <-- A-5
  \coordinate[above right=of A-2.west] (aux);
  \draw [arrows=-Stealth]
  (A-3.west)  to  node[pos=.5,below]{ναι} ++ (-1,0) |- (aux) -- (aux |- A-2.north west);
  \draw  (A-3) --node[below]{όχι} (A-4);
  \end{tikzpicture}