According to the Quran, God sent the Quran to "confirm" the Scriptures that had come earlier:

"To thee We sent the Scripture in truth, confirming the scripture that came before it, and guarding it in safety: so judge between them by what Allah hath revealed, and follow not their vain desires, diverging from the Truth that hath come to thee." (5:48)

"And when there comes to them a Book from Allah, confirming what is with them,- although from of old they had prayed for victory against those without Faith,- when there comes to them that which they (should) have recognised, they refuse to believe in it but the curse of Allah is on those without Faith." (2:89)

In 2:89 we also see that it confirms "what is with them".

According to Merriam-Webster, the word "confirm" means:

1: to give approval to : RATIFY

2: to make firm or firmer : STRENGTHEN

3: to administer the rite of confirmation to

4: to give new assurance of the validity of : remove doubt about by authoritative act or indisputable fact

Of course, to administer the rite of confirmation does not fit the context, but all the other meanings means that the Quran vouches for the earlier Scriptures. Scriptures that existed at the time of Muhammed: It approves of them (meaning 1), it makes them firm (2), or assures of their validity (4). 

If you look at different translations of the Quran, they all say the same concerning sura 2:89:

"confirming that which was with them" (Sahih International)

"confirming that in their possession" (Pickthall)

"confirming what is with them" (Yusuf Ali)

"verifying that which they have" (Shakir)

"which confirms what is with them" (Muhammed Sarwar)

"confirming what is with them" (Moshin Khan)

"confirming what was with them" (Arberry)

From http://corpus.quran.com/translation.jsp?chapter=2&verse=89

(The only translation of the above that gives a different meaning is Muhammed Sarwar, who adds a parenthesis that alters the meaning of the text. However, it is clear that this parenthesis does not belong to the text proper.)

Conclusion:

The Quran verifies and confirms the earlier Scriptures. Which means that it tells it's readers that these Scriptures, as they were at the time of Muhammed, were still valid.