Macbook 2013 - High Sierra lag

Hi,
since the last high sierra update my computer has a lot of lags, I want to reset it and reinstall Osx can someone explain me the step to succeed this?
Sincerely yours
Antoine

Hi !
Do all of this step and It will be probably work :
1. Start up your Mac pressing shift-Option/Alt-Command-R.
2. Once you see the macOS Utilities screen choose the Reinstall macOS (or Reinstall OS X) option.
3. Click Continue and follow the on screen instructions.
4. Select your startup disk and click Install.Your Mac will restart once the installation is complete.

Thank you your answer was really helpful