He can filter the physical debris later and the value will still come out good.
Is it desi ups or Chinese?
To check the voltage, you connect the Black(common/ground/negative) of meter to negative of battery. And Red(Volt) to positive of battery. Set the range to 20V DC and see what value it shows. Chinese ups cut-off the battery at 14.2v and later hold it at 13.6v for trickle charging.
To measure current, open the clamp and put inside the red wire of battery between the clamp and close it. Set the meter to measure Current at 10+A. The value you will see is the charging current.
If you battery feels warm and bulging from middle, it is overcharging. Another way is to buy a hydrometer and check specific gravity of electrolyte. A full charged battery will give around 1280 and anything below 1200 means it is not fully charged.
Expert can correct me if wrong.