The simple answer is to train. Depending on your age group, the requirements for getting silver varies. Come out with a feasible target for the number of push-up, sit-up and duration of 2.4km that you are able to achieve
Try to score higher for push-up & sit-up as these 2 exercise can be improved more easily. You can train everyday for these 2 exercise. Everyday, just set aside 1 minute each respectively for both push-up & sit-up. Set a target each week for the number of sit-up & push-up you are aiming to do in 1 minute. As each week progress, increase the target by 3 to 5 until you finally reach your IPPT silver target.
To improve your running you need to do a mixture of speed and endurance training. Try to run at least twice per week.
For speed training run 6 x 400m. Try to complete each 400m run as fast as you can. Rest for 1 minute then continue to run another 400m until you finish 6 rounds.
For endurance run, just run continuously without stopping for 3km. When you start training, run at a comfortable pace first, try to increase your speed as you progress