You have to use a (global) variable. You can call it enemyspeed for example.
Then you have to use "add to" instead of "set value". Because you dont want to set the speed always to 500. You want to increase it by 500.
It should look like:
Every X seconds - add to- enemyspeed - 500
You can also set a timer at the beginning of the layout, repeating every 5 or 10 seconds. Then instead of "Every X seconds" you can use the Timer condition.