Proud owner of my first Volvo XC70

[cattlecar]

If it has a wheel speed sensor code the traction control function of ABS could be engaging at odd times The nooise from that may be what you are hearing. The wheel speed being irregular will make transmission not shift properly or choose the correct gear range properly. Get the wheel speed sensor issue fixed and then see what is left not working right.

[hoonk]

i was driving and after a while driving he started again this weird changing of the gears, sometimes it was so hard that i thought the transmission will come through the floor

So today i went to the Volvo dealer, again and he told me that the transmission most likely will need to get replaced,

I found several posts pointing me in the direction to get a software update
Is there a way to do it myself?

Unfortunately you may have found out why the car was for sale

1st step would be to scan the control units, but software will probably not fix weird shifting, nor will a fluid change. It's possible that the transmission could be simply low on fluid, but there should be evidence of leakage.