Does LibreOffice support automatic updating fields?

I need a type of field so that I can place field all over the document, and when one of the fields is updated, all other fields will be updated with the same modification. This is for a template document so that the field can be labeled “company name” for example, and this label can be replaced with the actual company name, and, when this happens, every other identical field on the document will be updated.

Yes. This is most easily done via the bookmark / cross-reference facility. I will use cross-references in this example as the original field is highlighted, while a bookmark does not highlight the original field. Greater detail on the differences between these two method can be found in my post here.

  1. Highlight the piece of content you wish to be repeated.
  2. Insert > Cross-reference…
  3. Select Type of “Set reference”, enter a meaningful Name (e.g., MyVar). Click Insert and Close.
  4. Place the cursor where a copy is to be located.
  5. Insert > Cross-reference…
  6. Select Type of “Insert reference”, selection your variable from the Selection list, select Insert reference to “Reference”. Click Insert and Close.

Repeat steps 4-6 as necessary.

After setting this up use Ctrl-F9 to toggle between cross reference names and values. You can edit the source field and then use F9 to update all dependent fields.

In Step 3., you do not have to close the Fields window, you can move it out of the way, and do the "Insert Reference"as described in Step 6 in multiple places. You can also set new references (Step 3) without closing the Fields window.

As for Comment 1, ‘editing source field’ means, go to the first place in the text where you set the reference, and change the value in situ. Do not highlight or double click to change the value or you will lose the field and have to Undo. Append the new text, then delete the old text.

To change font and size, you have to do it for every instance, by placing cursor at end of field, then sliding it to the left a little to select the text. Any other method of selecting the text, for example, clicking on the text, will annoyingly send you back to the first place where you set the reference. After you have successfully selected the text, you can change the font and size.

To make a template, File > Save As Template opens a Template Manager window.
Click Save, and you will be asked for a name.
Give same name if you want to overwrite a previous template.
If you do overwrite, the window will show you two copies of the template with the same name until you come back to the window again.
I don’t know what the advantage of saving as a template is, over having a regular .odt file that you modify.