Write the date on the new battery.
Change it every 3 years,5 max.
Hope you have had it serviced in the last 9 yrs?, or a new backup battery at this stage could be the least of your problems.
Not having a dig,just stating a fact.
Follow gts's advice or replace the battery like for like as the battery details (Ah rating ) will be written on it because we don't know how loaded your system is.Eg more equipment on the system,then more Ah's.
Be safe.
e manno
September 2013