exactly as for the TS590 (well documented on the internet), but I will
show the photos I have made. It is not recommended to "open" the TX
completely. First it hardly serves a legal purpose, and second, you
have to remove an SMD resistor and this may well destroy your rig if you
are not used to SMD soldering. Fortunately, switching the "European" into
the "US" variant simply involves cutting a wire jumper and is easily accomplished.
So first remove the bottom cover. This is secured by 10 screws (6 on the bottom, and 2 on each side). After this, the TS590sg lies on your Desk as shown in the first picture.
The two pictures have a fairly high resolution, so if you download them, you should see every detail.
red circle on the first photo shows where you have to look, you will
find a wire bridge marked with "R968". According to the schematic, this
pulls down the TYPY line. This wire has to be cut.
On the second photo, you see the jumper (in the red circle) just after being cut.
Then, bring the bottom cover in place again and tighten it with the 10 screws.
Now, BEFORE powering on the TS590sg, press the "A<>B" button and hold it, THEN power up, wait few seconds, let the "A<>B" button loose and press it again. This makes a full reset of the rig to factory settings.
For example, if digimodes don't work after this, do not panic: you have to activate USB AF input through menu #69, for example, the default is to accept AF signals from the ACC jacket.