So basically there are two ideas in this sentence.
First, it is said that the examination called the “Combined Graduate Level Examination, 2013” should be held on on April 14th, 2013. It uses here the expression “is to be held” which means “should be held” and is grammatically correct.
Second, it says that the examination has been postponed, and is also grammatically correct.
However, the sentence is still not grammatically correct. The problem comes from how both part of the sentence are connected to each other. One way to solve this is to replace the word “is” that you highlighted with “which was”, or “scheduled”, as the comments suggest.
But there is also another simple way to make this sentence correct, simply by adding the word “but”, or “and” between both ideas, which then makes the sentence easier to understand. The word is then needs to be in the past tense as the action it describes was done before the rest of the sentence.
“Combined Graduate Level Examination, 2013” was to be held on April 14th, 2013 but has been postponed and will now be held on April 28th, 2013 (Sunday)
Note that there were other (small) mistakes in that sentence:
- The name of the examination should be surrounded by quotation marks
- The dates should be written as “April 14th, 2013” or “the 14th of April, 2013”
- And finally, the word now should be placed after the verb will.