The Fitbit Sense earns a 5 out of 10 on our repairability scale (10 is the easiest to repair):

Watch bands are easily replaceable (though not standard issue).

Standard screws are used prudently throughout the device.

The display-first disassembly procedure means screen replacements require only a few steps, but those steps are difficult and can easily damage the display.

The battery is buried under the main board, but it can be disconnected immediately upon opening the watch and is easy to remove once you get to it.

Components underneath the battery aren't modular, and will be very difficult to remove due to their strong adhesive.