How to clean this PC.

It could be a number of reasons that it is slowing down. Common problems are file and registry clutter, poor cooling or an old or failing hard drive. As far as clutter I would back up important stuff and do a factory reset. It can do wonders performance wise. Now if it is getting too hot and throttling you will need to pop it open and clean the fans and possibly repaste the heat sink. I couldnt find a teardown guide on here but did locate one at this address https://community.acer.com/en/discussion... Lastly, the hdd. A failing drive can cause similar performance issues. You could download a free disk benchmark to test its performance. If it is failing just follow the same teardown guide and replace the hdd. If it doesnt already have a solid state drive, get one. You will see your performance go up by leaps and bounds.