I have a Relic airplane clock and I do not know how to replace the battery, the number is RML0000040. On the bottom of the plane is Limited Edition and China Case

It's not easy. You're probably better off brining it to the jeweler unless you already have the tools. You'll need a precision screwdriver set. Under the clock there's a tiny screw. Carefully remove that to pop the clock off the front of the plane -- don't strip it. Then get a standard flathead screwdriver to remove the cover behind the clock. Just turn it so that the tabs on the cover appear in the little openings around the edge. It should then fall out. If not, use a tiny screwdriver to remove it.
Under there is the clock. Remove the plastic spacer. You'll see the tiny little battery, which is held in by a tiny metal arm. Move the arm and the spring behind the battery will pop it up a little. Use a tiny screwdriver to carefully remove the battery. The battery is SR626SW.
Have fun putting it back together! Whoever designed this thing is on my s-list.