It is mentioned above:

3x AAA 1.2V batteries with U-lips are required to be soldered onto the motherboard.

So yes, a soldering iron is required, and the battery must have lips (i.e. not the standard AAA batteries you can find in any shop).

I haven’t bought the batteries yet. They were overpriced in my region (with shipping costs). For now, I use the shaver with leads connected.

I did open up the shaver, but haven’t replaced the batteries yet; just wanted to ensure they’re identical to this guide (they were :-)).

The batteries I want are out of stock currently; I’ve set an alert when they become available. Meanwhile the shaver is closed (and fully operational, with short battery life) again. If I don’t forget, I’ll take a picture of the other side of the mainboard.

Removing the blade isn’t required. The device can be easily opened without removing the blade.