Based on the type of beacons you have you will have to follow different steps to open them up and then replace their specific batteries.
Indoor / Outdoor beacons
Remove the back cover using something sharp (e.g. a guitar pick) to find 4 AA batteries.
Battery type - AA
Keychain beacons
Open up the beacon from the bottom hinge near the keychain hanger using a pick or something sharp.
Battery type - CR2032
Pocket beacons
Open up the beacon by pulling it up from near the power button.
Battery type - CR2477
Long range beacons
Remove the grey rubber lining present on the back-side of the beacon using a pair of forceps. Remove the screw on the four corners and then lift the cover to find 4 AA batteries.
Battery type - AA