Try this code.
LED is on GPIO.2
Button must be pulled down to GND by a resisitor (10K).
And must be pulled high by the button (to Vdd).



Code:
@ DEVICE PIC12F683, FCMEN_OFF, IESO_OFF, INTRC_OSC_NOCLKOUT , WDT_OFF, PWRT_ON, MCLR_OFF, PROTECT_ON, BOD_OFF
OSCCON  = %01110001     ' Internal 8MHz osc.
DEFINE OSC 8
' ======= Common Settings =================================
ADCON0 = 0
ANSEL  = 0
CMCON0 = 7           ' Comparators off.
TRISIO = 0
GPIO   = 0
' ============= PWM Settings =========
CCP1CON =   %00001100       ' CCP1, PWM mode
PR2     =   250             '
T2CON   =   %00000101       ' TMR2 on, prescaler 1:4
CCPR1L  = 0
Speed var CCPR1L
Btn  Var GPIO.3  ' Assuming ; the pin is pulled down to GND by 10K resistor, and button pulls it high to Vdd.
' LED is connected to GPIO.2 (on board PWM module)
pause 10
Start:
    if btn then 
        if speed + 85 < 256 then    ' 0, 85, 170, 255, 0 
            speed = speed + 85
        else
            speed = 0
        endif
 
        while btn : pause 20 : wend
    endif
 
    pause 2
goto Start
end