The first thing you need to ensure is that a loose chain is the cause of problems with the garage door opener. Open and close the garage door for numerous times and observe closely.
Now you need to start work on the chain but before that, you need to establish a safe working environment. Completely close the garage door so that there is almost no tension in the chain. After the door is properly closed, disconnect the garage door opener from the power source to ensure there is no risk of an electric shock. You can accomplish this by shutting off the circuit breaker through which current is supplied to the door opener.
Modern garage door openers come with an emergency release rope which is used to release the garage door opener when a power breakdown or an emergency occurs. Tug at this rope to remove opener connections from the door. Place a step ladder close to the garage door opener assembly. Carefully climb the ladder and assembly a detailed look. You should be able to see a pair of adjustment bolts, with one adjustment bolt located on each side of the assembly.
Hold one adjustment bolt in your hand and with a wrench held in your other hand; slowly turn the other adjustment bolt counter clockwise to tighten it. Now tighten the other bolt in the same way. As you tighten the bolts, the chain will tighten as well. Make sure that the chain does not get over stretched because it may snap if that happens. When you have removed the slack in the chain, replace the garage door opener connections and reconnect the garage door opener to the power source.