I got this answer from Gigabyte support:
After confirmed with related department, the cover is removable; however, it may cause the hardware damage while removing it. Therefore, we suggest you to contact your supplier (where you bought this product) and let them help you to remove it.
That is all...
Well, she didn't please me. I don't understand that replacing a CMOS battery is literally a pain with such a premium and expensive product. Until I disassemble the entire computer, remove the motherboard from the case, the battery cannot be replaced. It's really incomprehensible to me that replacing a battery that takes 10 seconds will take me all day. My fault. If I had found out before, I would never have bought the TRX40 AORUS XTREME. Otherwise, the motherboard is great, it works perfectly. But the fact about the extremely complicated replacement of CMOS batteries is incomprehensible to me.