I would like to know if it's a project manager's job to help his/her team to improve their creativity and make them use it for the project. If yes, could you recommend some methods to me?
As you may already know, the project manager is the person responsible for leading a project from its inception to execution. This includes planning, execution and managing the people, resources and scope of the project.
Now managing the people part includes working with both the team and the stakeholders, as a PM you need to be able to listen, understand, then communicate in words, writing and deeds in order to get your work done.
And if the project requires the team to be creative and think out of the box, then it's your responsibility to plan and help the team to make it happen. You don't have to be the person who trains the team to be creative but you need to plan according to their needs how will they acquire the required skills to execute the project.
Regarding the methods, it depends on the project, do you need them to be artistically creative? or technically creative? or marketing wise or ...? Unfortunately there is no sliver bullet methods to be creative, you need to define the areas you need your team to be creative at; to be able to define a set of methods/activities that will help them to reach the skill level needed to execute the project.
To the extent practical, let the team members decide what they will work on rather than assigning tasks yourself. Allow them to experiment with tools and methods that are new to them. This might be easier on a scrum team than on a more traditional project. You can actually use scrum for work that is not software development.
An empowered team member is more likely to be a happier team member is more likely to be a more creative team member. Just be careful that their creativity doesn't lead them to spend too much time on things that are unrelated to the project deliverables.