Question: The more i study science and maths these days, the more i continuously question myself whether all the knowledge we have built up until now is in fact true or correct. 

But before being picky with what true is, like i know in science that knowledge will never be perfected, as scientific theories are just theories which claim to understand the natural phenomena in a better way than before, justified by experimental verification. But the scientific theory or knowledge built may always be superseded with a better scientific theory, which questions whether at any point in time what we know from science is really true at all? 

This is just one aspect of the confusion which i am having; however, more generally, if i were to say everything we know up til now is wrong, does this claim have any validity? 

In other words, if i were to say that everything we know is wrong, is this considered true?

Answer: 
This is a very common issue when dealing with science. Much of science's approaches to Truth (with a capital letter) is through [abduction](https://plato.stanford.edu/entries/abduction/), an approach which assumes the most likely hypothesis is true. If you read the linked SEP article, this is fraught with nuances, as you suggest.

Personally, I am a fan of radical skepticism, and the [Aggripan Trilemma](https://en.wikipedia.org/wiki/M%C3%BCnchhausen_trilemma). It's not popular these days to declare that one cannot truly know anything, but if nothing else it's an interesting foil to challenge the current tendency for other systems of thought to declare they have the Truth.

As an example, I find it is *very* common to assume that we know the true semantic meaning of a word or sentence. While we tend to permit axioms to be challenged, tested, and sometimes proven, the behavior of the language we are using is typically assumed to be true (for good reason: there's major issues that arise with a language defining its own semantics). What if the answer is simply that we are not using the same meaning of "knowledge?" I might be communicating about what I believe knowledge is, and you might be communicating about what you believe knowledge is!

An interesting follow on might be to question how one might live life if nothing can be known, using whatever concept of knowledge suits you. I believe there are some related things targetted by Zen Koans, which many state are designed to try to break you out of the way you think. Do we really need to know Truth? Myself, I find the answer I get about that question depends greatly on the individual doing the answering.
From a science perspective, we don't know that what we know is right. You could argue that at the most fundamental level the only thing that an individual knows is that they exist, that they are conscious.

It is also a mistake to say that science can "understand" natural phenomena. Scientific theories can really only describe natural phenomena. They do this by creating a model, often a mathematical model. For example, it is not correct to say that that anyone "understands" quantum theory. We have equations to describe the behaviour of very small particles that are moving at a large fraction of the speed of light. It turns out that compared to objects in our everyday experience, the behaviour of these particles is bonkers. This means that it essentially impossible to "understand" the behaviour of say, an electron in the same way that I "understand" the colour green, or the word "dog".

Scientific theories are models that describe natural phenomena. Our current theories do a better job at describing these phenomena than previous theories, and it is plausible that better theories will supplant (more likely, improve) our current ones.