We are talking about a default style, not a profile.
Follow the Modifying a Default Template instructions in the link I gave to edit your default style. when you open the template manager, the current default template will be indicated by a green tick; in my case I have created an additional template to act as default.
Right click on the ticked template to edit it; this will open the normal styles panel and allow you to edit any of the styles in the template.
You want to alter the inter-paragraph spacing; so first edit Default Paragraph Style and set the before and after paragraph spacings, in the Spacing tab, to your preferred value of zero. Note that Default Paragraph Style is the style from which all other styles inherit their values, unless modified in the child style. It should not be used for normal patagraph styling - this is where Body Text is used.
Next, edit Body Text and ensure that paragraph spacing is as required, as the setting there may be changed from the parent style.
Check any other paragraph styles you use or may use, and make any other changes needed. Finally, save the modified template. Note the advice in the link about saving in Template directories - this will place the template in a location that shouldn’t be overwritten by an upgrade to LO. This location should be somewhere in your home directory. or equivalent if you use Windows.
For more information, you might peruse this chapter in the Getting Started Guide.
Again, I think you mean template; if saved in your home directory it can be backed up as part of your normal system backups and restored after a Linux upgrade or re-install.
LibreOffice doesn’t seem to have a way to “apply” a template to an existing document. You could try the Template Changer extension.
Edit; It seems you have asked this question previously - Permanently disable “add space between paragraphs”.