How to Change Version of Bootstrap?

My BS4 works fine. I just wonder how to customize it with this set up.

when run mix.phoenix.server it never ends and loops with permanent warning =

warn: sass-brunch is taking too long to compile @ web/static/css/app.scss

UPDATE & Fixed : Brunch sends this not rreally explicit message also when he does not find some files…

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Many thanks. I hoped , I had found it 4 months back.