MySQL recharge

mysql How to add the data added last time and the data added now . such as ： Cumulative recharge , The last data is 100 element , Now I'll recharge 100 element , Now the cumulative recharge should be 200 element , How to operate .

At first, I recharged 100, Is shown as 100

Now I'll recharge 200, Is shown as 200, Not at all 300

What should I do? ？