how do I reduce 1 A4 page into 1 A5 page?
Use your XEROX’s zoom feature.
Seriously, though, you seem to give not that much details to be able to answer. What program (Writer/Calc/Draw) do you use? Do you need to change the page size of your document, or of its printout? Without these, your question doesn’t make much sense.
If you want to print it, use the “2 pages per sheet” option in the printer options.
If you want a new .odt file, re-format it with half sized fonts, reduced page size etc. It will take no more than two minutes - a shorter time that it took to write this.