update values everyday I have a table with 2 columns. Days is the days from Start Date to today.

How should Trigger be to automatically increase Days every day?

Start Date      Days  
19/07/17         19          
25/07/17         13
07/08/17         0
Triggers only work on the rows which are affected by the statement causing the trigger to execute. 

When you want to update a whole table, use a normal statement, not triggers.

When you have data, that can be easily calculated, don't store this in a table. Either you calculate it in your SELECT statement or create a view.

Example:  

CREATE VIEW view_with_day_difference_calculated AS
SELECT start_date, TIMESTAMPDIFF(day, start_date, NOW()) AS days
FROM your_table;


Then all you have to do is this in the future.

SELECT * FROM view_with_day_difference_calculated;