How do i clear the "service engine soon"

Disconnect the battery for a minuit and turn on the light's to power down any electrical power. Reconnect the battery and that should clear the "servie engine light soon",but if there are any DTC codes, the "Service engine light" will come back on.
On board diagnostic (OBD) was designed on vehicles equipped with electronic fuel injection so you can generally retrieve the codes yourself. No need buying a scanner or running to any parts stores to check the Engine Light. This system is called obd1 and applies to most vehicles made before 1995 for domestics and 1993 on imports.

For GM domestic vehicles made before 1995, the diagnostic connector is located under the dash panel by the driver side. To get the check engine codes to flash in your dash panel, use a jumper wire or a paper clip and connect terminals A and B of the diagnostic connector. Turn your ignition key on with engine off and the codes should start to blink. All codes should start with code 12 which is one long flash followed by 2 short flashes. This code 12 means the diagnostic system is normal and will repeat itself continuously if there are no trouble codes. Otherwise, code 12 will flash 3 times before flashing the fault codes.

the codes can be erased or cleared by disconnecting the battery negative terminal for 1 minute and reconnect. Just make sure to check your service manual in case you have electronic equipment such as radio or clock that needs reprogramming in which battery disconnection is not recommended. Finally after performing repairs on the culprit code, always go for a road test to confirm if the problem is fixed.

Good luck and hope this help, here is a link to the DTC codes. http://www.gmtuners.com/OBD1_DTC.htm

This is the emissions system malfunction warning light on any GM, you can reset it by disconnecting the battery for 5 minutes, but if the problem that caused it to turn on is not repaired it will come back on in a few days or about 20 miles.