This question raises an issue that most of the students preparing for JEE face. My suggestion to all of them, and you, is:
Work in parallel, rather than sequentially. You would be mistaken in thinking that you can postpone the revision of the 11th standard syllabus to some later date (which, in many cases, comes too late). What you need to do is to
do revision and cover new syllabus simultaneously. Practically speaking, this means that each day you need to give some time to covering new syllabus and some time to revise the portions already covered.
The logic behind this is simple: the more time you give to something, the more familiar/comfortable you get with it. Its like a guitar player's situation. Theoretically speaking, one can learn to play the guitar in, say, even two weeks or a month. But that learning would be very "rickety". It will take years of practice before you can even begin to think of playing like Eric Clapton.

The same holds true for JEE preparation. The more time you spend revising the syllabus, the more familiar you get with it. That's why my advice to you would be to develop a schedule that has revision and syllabus-coverage going simultaneously.
Hope this helps! For further clarifications, you can always ask again.