Because the gap between the screen and watch body is so thin, a sharp blade is required to separate the two. Read the following warnings carefully before proceeding.
Protect your fingers by keeping them completely clear of the knife. If in doubt, protect your free hand with a heavy glove, such as a leather shop glove or gardening glove.
Be careful not to apply too much pressure, as this might cause the knife to slip and cut you, or damage the watch.
Wear eye protection. The knife or glass may break, sending pieces flying.
Using a curved blade minimizes the chance of scratching the case or cracking the glass. Only pry with the curved section of the blade, and not the tip or flat section.
Place the curved section of the blade in the gap between the glass and case on the lower edge of the watch face, and press firmly straight down into the gap.
Be very careful to maintain complete control over the knife—once the gap opens, if you're pressing too hard on the knife it may slip in and cut the battery.
This should wedge the gap open and cause the glass to lift slightly up from the case.
When the glass has lifted, gently rotate the knife down, opening the gap more by pushing the glass up.
There are two cables connecting the screen to the inside of the watch, near the top left corner. Be careful when prying or you may damage these cables.
Pry slightly up on the right side of the screen, to free it from any remaining adhesive.
Pry up on the left to free it as well—but do not attempt to remove the screen as it is still held in place by two cables.
If you can see the top layer of your Force Touch sensor among the display adhesive, it means the two layers of the sensor separated and you're going to need to replace or repair it.
iFixit screen and battery repair kits come with a replacement Force Touch sensor, so if you got one of those, don't fret.
The top layer of the sensor may be adhered to the back of the screen—if so, push it back down and separate it.
Place the watch on an elevated surface, at least 1/2" or 1 cm tall—a small box or the edge of a book will work great. This will allow the screen to hang down vertically and give better access to the battery.
Be careful not to bump the screen or strain the cables while you work.
Use scissors to cut one of your opening picks to about the width of the battery. Try not to leave any sharp corners.
Insert the modified pick between the right side of the battery and the case.
Use constant, steady pressure to slowly pry the battery up, separating it from the adhesive securing it to the system board.
Be careful not to deform or puncture the battery.
On the larger (42 mm) models, it's possible to accidentally pry at the system board underneath the battery. Insert the pick only far enough to get underneath the battery, not the system board.
If needed, apply a little high concentration isopropyl alcohol (90% or greater) around and under the battery to help weaken the adhesive.
Do not attempt to remove the battery as it is still connected.