SQLite Android - Updating (adding up) existing value with new one

I am new to android and i am trying to use these code to update an entry's value:

"Update " + TableIncomeCategory + " Set " + Income_Cat_currentAmount + " = " + "'" + Income_Cat_currentAmount + "'"
                        + "+" + "'"+tran.getAmount()+"'" + " where " + Income_category_name + " LIKE " + "'" + tran.getCategory() + "'");

I am trying to add up the current amount (Income_Cat_currentAmount) with the new value (tran.getAmount()) the code is running without crashing but every time I run it, the new value replaces the old one rather than adding up together. Can anyone tell me why and how to solve it? Many Thanks!!