Hello Rberkness,
Thanks for your question about adding Google Tag Manager into your site. Your concern about it getting overwritten is well-founded, as an update of the theme will remove the entries. I think the easiest way to get the code into your theme is to go into the WordPress Administrator, then click on Customize >CSS/HTML Editor. You can select the head.php template file here and edit it. There’s nothing that openly states “<body>” tag, so you will need to determine where to put the other entry. It will likely be one of the content pages. I’ve tried looking up installation suggestions but nothing on where to install it in this type of theme. You may need to contact the GTM developers for advice on how to install it into a WordPress theme using Bootstrap style theme.
If you have any further questions, please let us know.
Kindest regards,
Arnel C.