Back button look bootstrap which one to use?

I have a multi page web app where the user first selects some information and uploads a file to import, then the file will be sent to the back end for processing. After that the results will be presented to the user in a new page. There the user can make some selection on what years to include in the import. On that page upon inspection the user is not satisfied he/she can go back to the file upload page and upload another file / use other options. The normal flow however is to press submit so these things will be imported. On that page i have a back button. The question is how should it be presented?

The action is not dangerous since it's just a matter of restarting.

I'm quite limited to the buttons available in bootstrap css

I'm choosing between warning, default or link. And is leaning towards link, since it's quite a neutral one, and have the intention of linking the user to the first page (they could as well just begin from scratch by going to the import link directly)

Do you have any other things I should consider?