A simple sketch just for these 5 buttons functioning will be appreciated.
Not possible without a lot more information.
A slider can take on any value in it's range. So, it's range is typically set to match the necessary output. In the case of a slider controlling a servo, the range of the slider is typically 0 to 180. Whenever the slider is moved, the new value is sent, and the receiver moves the servo to that new position.
Buttons on the other hand offer discrete events. YOU have to decide what the servo should do when a button is pressed. Should the value to be sent be increased by 1? By 5? By 14? By 22? Decreased by 7?
Once YOU decide that, then buttons can replace sliders. You also need to decide if you are going to keep track of the servo position in the PC application of in the Arduino. If the PC keeps track of the value, then it should send that value every time a button is pressed.
If the Arduino is keeping track of the value, then you send either a delta (+5, +12, -7) or an event (UP, DOWN, LEFT, RIGHT, WAVE), and program the Arduino to do something with either the delta or the event.