1) Reverse the UPS to understand how battery were wired internally
2) make a nice terminal to connect the new battery externaly (that UPS was made to have internal smaller battery)
3) make a dry floor for the battery
4) wire the battery together and to the UPS
5) fire up the system and pray that nothing catch fire
6) add a bit of monitoring (smoke detector, flood detector, get the state of the battery from the UPS
7) connect it to the house electric network
This is super dope!!!