What are the best fastest steps to raise up the spirit of a frustrated team and has a sense of defeatism?

Sometimes teams can be frustrated if they fail to achieve certain targets. In order to boost their spirit, you have to make several intermediate goals. Team will feel a progress and they will boost their confidence gradually when they achieve those intermediate goals. They will start to think positively hence you can guide them towards your ultimate goal. And it is very important to set realistic goals. 

A revised target plan and attached to that bonuses on achieving more than the desired target. this might be helpful to steer them away from defeatism

Hello.

I would start from a question - what brought your team to such condition.

Find the origins of the problems and heal them.

Human being function on emotions. Before exposing any new plan, I will get close to people, hear what they have to say and show empathy. Listening is the first quick step to take. Then trust needs to be built. This is the longest step and in between there is a set of actions you can take to demonstrate intentions and goodwill. There is no quick fix that miraculously will put the moral back to higher ground, but what is determining is a genuine intention backed up by actions. 

due to Newton law, for each action force there is a reaction force, equal to the first force but in opposite direction , to maintain equilibrium

so if your team feel defeated , try to open a new page - celebrate with them a party and get ready the next day to put new decisions, new plans, new strategies, new ideas, new way of cooperation ....