If Bible is altered, then why the quran instructs Muslims to take reference from the Bible?
Good question.
That is the reason to learn and understand any document that is written in its original language or from someone who is well versed in that document.
The word that is mentioned is 'Injil'. Injil is the book and teachings of Jesus.
Yr book is called the bible from the Greek word biblios. Which simply means 'The Book'.
Jesus in his lifetime did not authorize nor approve of this bible. It was written by anonymous people who very likely learnt/copied it from his disciples.
Over the centuries, the originals have been lost, and the current ones have been translated, mistranslated, changed, modified, reviewed, verses put in and out, until glaring mistakes, contradictions, errors are found in it. You must have encountered people questioning the priest or pastor, and he being unable to answer, replies, just have faith!
Even though there is some truths that we Muslims can agree on, largely, we reject it as we cannot say which are the authentic verses.
Thus, if you find the very original, we are confident it will align with the Quran, and be without errors and contradictions.
For eg, human hands have modified Jesus teaching that some gospels are not canonized like the Gospel of Barnabas and many more others.
In fact the people who followed Jesus' early teachings were known as Nazarenes. Not christians.