To change the chain on a Stihl MS 170 chainsaw, follow these steps:

First, ensure that the chainsaw is switched off and that the spark plug wire is disconnected from the spark plug. This will prevent accidental starting while you work on the chainsaw.
Use a wrench to loosen the nuts on the side panel of the chainsaw. Once the nuts are loose, remove the side panel to access the chain.
Carefully remove the old chain from the chainsaw. You may need to loosen the tension on the chain to remove it. To do this, use a flathead screwdriver to adjust the tension screw on the chainsaw's bar until the chain is loose enough to remove.
Clean the chainsaw's bar and sprocket to remove any debris or dirt that may have accumulated. This will help the new chain to run smoothly.
Place the new chain onto the chainsaw's bar, making sure that the teeth of the chain are facing in the right direction. The chain should sit snugly against the bar, with no slack.
Reattach the side panel of the chainsaw and tighten the nuts with the wrench.
Use the flathead screwdriver to adjust the tension on the chain, ensuring that it is neither too loose nor too tight. The chain should be tight enough that it doesn't sag, but loose enough that it can move freely around the bar.
Finally, check the oil level in the chainsaw's oil tank and refill if necessary. Proper lubrication is important for the chain's longevity and smooth operation.
With these steps, you should be able to change the chain on your Stihl MS 170 chainsaw and get back to work. It's important to remember to handle the chainsaw with care and to wear appropriate safety gear, such as gloves and safety glasses, when using it.