All it does is look if the script is being sourced in your profile:
$profileLine = ". '$installDir\profile.example.ps1'"
if(Select-String -Path $PROFILE -Pattern $profileLine -Quiet -SimpleMatch) {
Write-Host "It seems posh-git is already installed..."
return
}
Since it seems to have the installDir as the folder from which you run the install script, merely upgrading the folder with the newer version of posh-git should give have updated the files already.
If you are not comfortable with that, just remove the line that sources the profile.example.ps1
in your profile and run the install again :)