“Since man’s days are determined, the number of his months is under your control; you have set his limit and he cannot pass it.” ~ Job 14:5 

~ God has set a limit on the number of years of life that you will live. You will not live less than nor more than that limit.  

So, you cannot change the number of years that God has set as your limit. If 80, then it will be 80. If 55, then 55 it will be.

Now suppose you decide to take up running to improve your health, and also to lengthen your life. Suppose you die at 75 and just before dying, you estimate that had you not taken up running, you would have died at 70. Then when you go to Heaven, you will find out that God had set your limit at 75.

Or suppose you live an unhealthy lifestyle with no exercise, and so on and then die at 70. Suppose that just before dying, you estimate that had you lived a healthy lifestyle, that you would have lived up to 75. Then when you go to Heaven, you will find out that God had set your limit at 70.

So, from an earthly plane, you can change the length of life.

From a more heavenly (or really philosophical/theological plane), God set your length of life, and you cannot change what He has set for you. If God set your limit at 100, then a 100 it is. From this plane, you cannot change it.