On this unit, the likely culprit is the battery and that it needs to be replaced.

With that, you may want to think about the pros and cons of replacement versus buying an entirely new unit.

This unit is a relatively low end ups and the surge supression uses MOVs that wear out with repeated absorption of energetic spikes. That's why the better (and higher cost) UPSs and surge suppressors don't use them. They're not really that good anyway due to their relatively high transient impedance. The better equipment uses a "transorb" -- basically a back-to-back high voltage zener diode optimized for low breakover impedance and high instantaneous power capability. Unless the transient exceeds the design parameters of the circuit, these do not wear out and do not degrade. That being said, I will assume that you would likely replace this unit with a similar one.
The flashing may not be the battery at all and may be the charging circuitry, thus your new battery may not live very long (half the life that you would expect.

Many batteries out there are refurbs and have issues as all used parts do.

If you contact APC, they will likely give you a 35% discount on purchasing a new product.

The difference in price in the battery replacement vs. ups replacement at this level may only be a few dollars more.

I promise that I am not a UPS salesman, I just deal with many on a daily basis; therefore I am giving my two cents to try to thelp you avoid problems a year from now.