Ill start off by saying that its a really bad idea to overclock smartphones as they already without being overclock heat up alot. You can very easy end up with a burnt out cpu, battery or both. However its your phone and your choice so this is how you do it:
You will definetly need to root the phone, follow this article to do that (https://www.cnet.com/g00/au/how-to/how-to-easily-root-an-android-device/?i10c.encReferrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS5hdS8%3D)
Then download setCPU app off google play and change it from that.