my iPhone is lagging

No unfortunately, No official (correct if wrong others) way to do this and avoid any third party utilities that are unsupported by Apple that could do more harm than good Including bricking your phone if it fails part way through.

So, iPhone 8 is the oldest model to support 12.6, so that’s going to test it a bit. The battery of such of from the original will have aged a lot by now.

• you can close apps by double clicking home button swiping up- the effect of this may be negligible
• free up space - in case your phone needs some for memory etc
• Turn off background app refresh for unnecessary apps
• Check battery in settings to see what Apps are power hungry
• see battery health in iOS*

*consider an Apple battery service

(a trip to an Apple Store if one close by might not be a bad idea regardless)