Font change simultaneously

I have a multi-page text in LibreOffice Writer which has quite large fonts. I’m wondering how can I change the font of ALL footer texts at once?

Depends on how your text has been formatted.
Your footers, like all other LO Writer text, will be using a paragraph style. This is very likely to be the Footer style that comes pre-formed with the LO installation. But you can check by putting the cursor into the footer and opening the Styles and Formatting list by pressing F11 or selecting View > Styles. The style being used will be highlighted.
If you have have not used direct formatting then all you need do is modify the footer paragraph style (or whatever style they are in) as you wish and all the footer text will change.
To modify the style right-click on the its name and select Modify to make your changes.
However, if you have already modified the footers using direct formatting, then use Format > Clear Direct Formatting to remove this hindrance to a universal change from the footer style.
If your text has been created using more than one Page Style there might be a little more to be done, but I hope the solution will be obvious.
And sorry for the repeated modifications I have made to this advice. I confused myself of thinking about footnotes.

Hi! I’m sorry, but unfortunately you have given a great answer to a spammer who made the legit-looking question to later change it to a spam. I suppose that the question should be removed, which would mean your answer would got removed, too. What do you think?

My 2 cents:

I, for one, think @Platterbaff’s answer is valuable and worth keeping.

Perhaps you could edit out the link in the original question, accept the answer and close the question as ‘Closed. Correct answer…’ etc.

@ Mike Kaganski. I hope our aim is to make Ask LO a source of good information, particularly for newcomers learning to use LO. So I am quite content to accept the advice of those with more experience than me.

I suspected the question might have come from someone intent merely on wasting our time. But it relatea to problems I have helped beginners with, so it seemed worth a few moments trying to produce something that would encourage intelligent newcomers to explore what LO can do (and, I now notice, I failed to lay out these suggestions as clearly as they should be).

But then, as Mike points out, the original question was changed to spam and I don’t think the newcomers I have in mind would have bothered to read the answer.

Thanks for your help with this.