You cannot reduce your crowdfunding target once you have started your campaign.
When you create your project, you choose whether to have a Keep What You Raise, or All Or Nothing crowdfunding model. You can read about the difference between them here.
If you select Keep What You Raise, then you need not worry - you will receive any of the funds you raised regardless of whether you hit your target or not.
If you select All Or Nothing, then it is important to plan your project carefully and then once happy with your fundraising goal, commit to it and develop a campaign that will help you acheive it.
If after starting your campaign you feel like the target is too ambitious, we can change the campaign to Keep What You Raise provided that you post an update to explain this change to backers, including how you intend to spend the funds raised.