Well you could just replace the battery except it may be that the battery is fine, it may be that some other fault is causing the battery to prematurely fail. You need to prove what is causing the battery’s loss of charge?
Check the battery’s initial level of charge with the ignition and all electrical circuits switched off the battery voltage should be greater than 12.4v, if below 12.4v then fully charge the battery. Calculate the number of hours to fully charge a flat battery by dividing the battery’s Amp hour (Ah) rating by the battery charger’s charging Amp rating and adding 10%. The slower you charge it the better. With a fully charged battery and multimeter in hand you can then test the battery.
Check voltage drop with the engine off, ignition on and all lights on expect between 12.5v to 12.8v slowly dropping to no less than 11.8v, if there is a fast drop to 11v suspect the battery.
Check electrical loading with the engine at 1,500 RPM and all electrics switched on expect the initial level of charge + 0.5v or greater.
Check cold cranking while cranking the engine on the starter motor expect battery voltage to remain above 9.5v.
Check standby current drain this varies from vehicle to vehicle so check the vehicle manufacturers specification for your model, typically drain will be something less than 0.5mA but with some vehicles can be as high as 2mA.
If the battery proves serviceable check the charging system. With the headlamp on high beam and the engine running at 5,000 RPM battery voltage between 13.2v – 14.2v is OK, if it is less than 13v suspect a stator fault, if it is more than 15.5v suspect a regulator/rectifier fault.