At the rear wheel gear, the gear does not change from the 7th gear to the 6th gear when i change the gear in the handlebar once. But the next time i do it it changes. So, if the manual in the handlebar shows 5th gear,it is actually the 6th gear. What should i do?
It sounds as though your cable needs to be adjusted.
On the rear dérailleur there will be a barrel adjuster where the cable enters the mech. You can increase the cable tension by rotating the barrel adjuster anti-clockwise and loosen it by (you guessed it!) rotating it clockwise.
By getting the cable tension right you should be able to get more accurate shifting.
Park Tools have a great guide to this on their website. You want section 6 on that page, which deals with indexing (the process of achieving accurate shifting).