Current our theme enable login/register links on header. If you want to change them to your custom links, please follow guide:
Firstly, check your setting of theme what header you are using. Ex: Header 11
Second, open file header-*.php at /wp-content/themes/linstar/templates/header
Ex: header-11.php ( as Header 11 on setting)
Then file to lines as bellow:
Change your link on href attribute of <a> tag then save to see result.