Need to adjust the timing for convertible top 1999 BMWi

It is (normally) not adjustable. If a mechanic told you the car is out of time, check with another mechanic.
If you have a damaged reluctor wheel, the crank position sensor can get mis-readings and cause the car to fire out of time. I recently saw a Lexus that had its reluctor wheel damaged during a timing belt replacement. I have heard of similar mis-haps on BMWs. A related problem is damaged shielding on the crank position sensor lead. Such damage alows electrical interference to get in to the ignition trigger signal from the sensor, and can cause mis-fires.
If you put in an aftermarket performance "Chip" in your ignition computer, it will change the timing program. Such chips are illegal in most states and may be detected in an emissions inspection. Is that what happened to you?

the timing an not be adjusted as this is eset in the control unit of your engine management what is your problem and why do you want to set the timing