I want to switch PhD programme
Yes you can change if you want. Your advisor will probably be unhappy about this, but you have to do what's best for yourself. 

And yes you can tell other professors that your position is not what you thought it would be, so you are looking to change. Yes it won't be the best story for them to hear, but it isn't universally poison either. I presume you don't exactly want them to ask for a reference from your current employer (on that note, probably should be quite careful your current advisor doesn't find out until your plan is set). 

But finally consider that many (if not most) people probably feel overwhelmed, like an imposter, in over their head, etc. when they're six months into grad school. And it's probably never exactly what they expected to be doing. So just be sure this particular position is really the whole problem.