Is there any possible way to go from alpha 18 to alpha 17 e.xt?
It’s not currently possible unless you made a copy of your Stonehearth folder (since now we are moving to A19, A17 is not accessible anymore).
When playing with Steam, you can switch between previous stable Alpha and current unstable Alpha. Once a stable Alpha is released, you can’t switch to the previous Alpha anymore.
But why do you want to go back to A17? 
Ah wanted to play on the sacred grove map and other mods 
That’s a good reason 
They should update the mods at least to A18, since in A18 there are some huge changes 
(Normally the mods don’t break that easily, it depends on what they add to the game).