Q40 n V 28 .score 560.how to improve to 680 in 20 days

Well, seems like you have some time at hand before the deadline for postponing your exam.

Try the following, may help!

As you have 2 weeks to decide whether to postpone your exam, you can well try to improve your verbal score, give another CAT and decide based on the improvement you see.

A tip: Aiming for 680 won't help! Aim for a 700 so that 680 becomes easy catch!

As far as verbal is concerned do the following:

From the tests that you give or the questions you solved gauge your problem area.
eg. If your SC is weak, try to pinpoint the concept that baffles you most. e.g if it is paralellism, revice the concept from MGMAT SC book and try solving questions from OG and the moderate and difficult questions from the forum every day. Try doing atleast 20 questions on this topic and increase the difficulty level each day. Keep in mind to go through all explainations for the questions, they help!

Alongside keep practicing your strong areas as well. Fewer questions but a mix of easy , moderate and difficult levels

Courses:

Try the E-gmat free videos or the ones by Knewton. I personally like the E-GMAT ones!
or any other material that helps to clear your basics!

GMAT Club threads will give you the headstart!
You can browse through the sections that you seem week in, understand concepts, solve problems etc.

Later when there are exactly 8-9 days for your exam, give another mock CAT and see if you are in a better position to appear! Else postpone your exam!

A boosting tip: MGMAT CATs usually have a lower score than actual GMAT for many people!
This may encourage you to try and improve in a week!