Battery replacement

Morning Im not sure how good my battery is The car has been standing for three weeks and trying it yesterday ,the battery wasnt good enough to turn the car over If I replace the battery,will it upset the alarm system Len

Wire another temporary battery in parallel whilst you change over & the car won’t know anyones been meddling with its private parts. Do observe the polarity & avoid sparks as a battery creates hydrogen when being charged. I have first hand experience of this whist my dad was testing headlamps by grounding the rim & providing positive to the contact using a long screwdriver. The results were inevitable with acid & bits of battery case all over the place & dads face. No more need be said.
M

Probably just needs charged...