Can I replace the battery in my UPS with a larger ones

[Jugernot]

I've got a UPS with two 6v 10Ah batteries which is a 520va model currently. Does anyone know if I can put new batteries in place of the old ones to increase it's rating?

I'm unclear of what exactly limits the capacity of a UPS other than the batteries. Does the circuitry inside the units only allow a certain amount of current output?

BTW, This is an older APC Backup 520va UPS. The data stamped on the top of the case is from 1991.

Thanks