Nixing McDonnell and Milne straight away would be a start. Which shouldn't be too hard because even aside from their personal views, neither are very good at their jobs.
He'd also need to make clear that Labour is first and foremost a parliamentary force, that any "social movement" is there to support and add to it, not supplant it or fight against it. Give up on the Trident opposition, it serves only to accentuate the splits in the party and is never going to succeed, especially given the unions. Make genuine effort to rebuild bridges with MPs, even if they're currently unwilling to serve in the shadow cabinet. Stop Richard Burgon.
In short, tone down the George Galloway aspects of his persona, amplify the Bernie Sanders ones.
Not sure I am indicative though, I'm one of the 4.5% after all
@Frosty is more someone that has lost hope after initially being positive, so he may be better to ask.