The Labour Party is a disaster at the moment. Never mind that many people don't trust politicians; right now, they don't trust one another either! Right now, there doesn't seem to be a clear path back, so what can the PLP do, to turn public - and members' - opinion around? Here's my advice...
Stop falling into traps being laid by the government's advisors, traps that are designed to magnify minor - or major - disagreements within the Labour Party; get good advice on strategy and engagement with the public/membership.
Be the team that supports and helps the leader to stand at the front of a strong and united army of hope against policies that harm the poorest and champion the rich.
Do your damnedest to demonstrate that your key priority is the people who put their trust and faith in you to properly represent them in parliament. Show everyone what badass opposition looks like! Then and only then, might you have demonstrated that one of you, (incumbent leader or not) is able to step up and lead a united, forceful and effective Labour Party into government.