I found a hack to do the above but I am sure there is more elegant way to go about it…
From within a /lab interface (and with the /dash endpoint open in another tab),
- 
I simply updated my
app.pyfile with the code above
 and saved it (without committing any changes) - 
I open a terminal and looked up the
PIDof thepythonprocess running the dash app using 
ps -fC python
and then killed it (a new process gets automatically spawned)

- I refreshed the 
/dashtab and my app updated in a running session !