Find a way to open this lock to replace battery

take the lock to your local home store that carries this line of locks. they will remove the core and put it in a SmartKey Reset Cradle. this returns the lock to an unprogrammed state.

there is no way to do this yourself without disassembling the lock core, and there's about a bazillion point 2 pieces in there.

This looks to be like a schlange or kwikset lockset commonly found at Home Depot. You can easily re-cylinder this, but it isn't obvious how to get the lock off.

My solution was as follows:

1. Rotate the lever to the vertical position. There will be two screws hidden under it.
2. Remove them, and separate the lock into two halves.
3. The lock-half has a cover on it. Remove it.
4. There's a screw that holds the cylinder in.

You can also get a lock/lever combination, as well as new doorknobs -- in many cases you can re-use the existing deadbolt and striker latch and just change the visible hardware.