If the numbers are written as words, i.e. one, two, three, I do not think there is any way to convert them.
If the cells were formatted as text cells prior to the data being entered, I do not think you can convert them to numbers. It was an interesting question so I just tried two things.
1. I tried reformatting the cells that had been formated as text cells but had a number in them. It cannot be done,.
2. I tried creating a formula such as the one mentioned by saitam and that did not work.
The key is to keep all cells formated as number cells. You can enter text into a number formated cell, but you cannot change it. Now if you put 5X in a cell and then write a formula of =Sum(5X*2), you would get the answer 10 X. But if you tried to have one cell with X and another cell with Y and then wrote the formula =Sum(A1*A2) you would only see the formula.
Short answer once entered as a Text character, it will always be a text character.
I am not an expert on Excel, but have used it a lot. I stand to be corrected. It is very possible there is some command I do not know and would like to hear it if anyone knows.