Here is a great video on how to do it safely: https://www.youtube.com/watch?v=zCA0Sw8h... Whenever equipment is sitting unused over several seasons or especially several years, the gas should be drained as it can create a lot of costly damage to the motor.
There is an entire troubleshooting guide for tightening the cable as well as other issues that may arise here: https://lawntoolslist.com/best-self-propelled-lawn-mower-reviews/ and here: http://www.ehow.com/how_8758641_adjust-cable-toro-selfpropel-mower.html