I’m currently doing a PhD in neuropsychology, however 6 months down the line I have fell out of it. I realised I’d like to do research in a field such as health psychology or clinical psychology. Would it be possible for me to get picked for a postdoc position in one of those areas coming from a neuropsychology background? (Considering that I don’t want to be in a postdoc that has something to do with neuropsychology in any way, but more like developing a behavioural change intervention or researching about mental health without digging into the biological aspects of it) , or should I just switch to a different PhD?
I'm not going to answer your question. Rather, I want to note that this is one of three questions you posed -- the two others being found here and here, all posted within the space of a few hours -- that paint quite a picture of dissatisfaction with your current situation. It's this overall state on which I want to comment.
It's common for PhD students in the early stages of their candidature to have doubts about many number of things. The questions you pose are among those issues -- satisfaction with their project, supervisory concerns, unclear targets, future prospects following graduation, waning interest in the original topic. You're not the first to experience these uncertainties and you won't be the last. To be honest, though, I'm not sure that a group of anonymous, well-intentioned individuals on a forum such as this can do anything more than say one of two things: (1) buck up and get on with it, or (2) quit while you're ahead. Importantly, I'm not sure what you want to achieve here.
The only way to resolve these issues is to discuss these with people who can help you reach a decision. My first response would be for you to discuss these issues with your supervisor, but it seems that you have some issues you need to resolve in that arena. I suggest that you seek support from your university's graduate school, your department's academic adviser, more senior post-docs or PhD students or your peers. Your partner, your friends, your parents and siblings can also offer valuable advice tailored to you.
Once you have received their counsel, then all that remains is to act.
Your greatest enemy here is time.
I wish you the best of luck, but more than that, I wish that you attain the degree of clarity you need to act.
In a post-doc, you're a hired gun. You're getting hired to do a job. If you're qualified to do that job, you're not really switching fields -- you're extending your research interests. If you're not qualified to do the job, you probably won't get that post-doc.
I strongly disagree that it is a bad idea to switch fields, as I switched from mathematical modelling of cardiac electrophysiology to genetics of bowel diseases and this has worked out very well. Although this question was about biomedical science, not psychology, I'd like to refer you to it anyway, as in the accepted answer, I discuss the pros and cons of switching to a different field after your PhD. Switching fields within biomedical sciences between PhD and postdoc?
Your question is not a duplicate of this one, but my answer applies to this one too.