The only way to make any iPad cellular-capable is to buy it that way; there is no separate upgrade after purchase. You would have to exchange your WiFi-only iPad for a cellular model. That could have been done during the first 14 days, as Zachyy and Johnathan Burger said (assuming you bought it directly from Apple).
At this point, the best you could do would be to buy a new cellular-capable iPad (or whatever model fits your needs now). Depending on how you purchase the new one, you might be able to trade your current iPad in for credit against the purchase, or you can sell it on the open market. (Don't expect to get back everything you paid for it either way. It is now used gear.)