I want to loss weight, am doing 45 minutes uphill walk (apprx 2.5 km) and then a 30 minutes downhill. If I switch to biking of 1-2 hours will it be more effective then uphill walk.

Cycling can be an effective cardio method, and I have lost a lot of weight cycling (fast cycling can burn 800 cals an hour) but you also have to look at your diet. If you finish each cycle with a peanut butter and banana sandwich, you'll be fit, but you won't lose weight.

The secret of losing weight is to loose calories.
I work out on stationary bicycle and burn 900+ calories in 45 minutes.
I have lost 12 kgs in 50 days with cycling and a bit of dieting.