Here is one way to do what you want:
In loop, test to see if one of the buttons is pressed:
if Button 1 is pressed then decrease a variable that holds the servo angle, checking to make sure the angle does not go below 0.
If Button 2 is pressed then increase the variable holding the servo angle, checking to make sure the angle does not go above 180.
Write the servo angle to the servo.
Delay 20 milliseconds (this is important to debounce the switch)
You can have another variable in your sketch that holds the amount used to increase or decrease the angle on each pass through the loop. The bigger this value the faster the servo will move whan a button is pressed.