WARNING:

if you plan to do this yourself, then any damage you
may cause to the ECU will be your own responsibility !

If you have never worked with a soldering-iron before, don't even think about doing this.
If you're unsure, get an old circuit board of an old computer or something similar and
start to practice (de)soldering a few components/chips
Once you think you have a feeling for it,  you can give it a try.

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Remove both top and bottom cover of ECU, each cover is held by 4 screws on each corner.
It may be neccessary to heat the screws with a soldering iron, since it can be quite difficult to get them loose
(you might have to drill some screws out)

Once you have removed the covers you'll see the main circuit-boards of the ECU.

The EPROM is on the motherboard (the bottom one), so you'll have to remove the daughterboard first.
This one is also held by four small screws on each corner and these can be very hard to get out as well.
If you have to drill one or more screws out, be careful not to damage the circuit-board(s)
Make sure you don't leave small pieces of iron on the boards !! (use a small brush to clean it once you're done)

Once you have removed the screws you can flip the daughterboard over.
Be careful not to damage the flatcable that attaches it to the motherboard.

You now have a clear view on the EPROM:

Now comes the part where the (de)soldering-iron comes in..
Below you see a close-up picture of the bottom of the board where the EPROM is soldered to the motherboard.
You'll have to desolder all 28 pins of the chip.

TAKE YOUR TIME !!
use a good quality desoldering iron...it makes things a LOT easier.
don't hold the iron to the board too long or it will overheat/damage things
If it takes you longer then expected, let the board cool down every few minutes.

Once you have desoldered all 28 pins you can GENTLY flip the old EPROM out with a small flat piece of strong plastic.
push it under the EPROM and try to wedge it up very gently..
It might take a little effort, because sometimes the clearcoat that is used to protect the board will make the chip stick to the board

If it doesn't want to come out in any way, check if all pins are desoldered and as clean as possible

If it still doesn't want to come out, you can use the 'cutting' method as a last resort....but...
There will be now way back if you cut the chip out... the stock chip can not be used anymore !!!
(So if you don't want to go this far, solder the pins back to the board and put everything back together.)

The Cutting method :
take a sharp stanley-knife and cut the pins on top of the EPROM
once you have cutted all the pins you should be able to flip the chip out and remove the leftover pins from the motherboard

Once you have the chip out you can put the EPROM-socket in and solder it into place.
It's best to use a soldering iron with a small tip for this..and once again take your time !
If it takes longer then expected, let the board cool down every few minutes.
Cleanest method to solder is to hold the iron against a pin of the socket and then add a little solder to it and let the solder 'flow'

Once done put the old or upgraded chip into the socket..
The little gap in the chip should point to the right (to the red LED's direction)
To be sure clean both mother and daughterboard with a small brush and
screw the daughterboard back into place and put the covers back on.

Now connect the ECU back into the car..just put the cable in, don't put everything back together just yet

start the car and see if it idles ok.
If it idles as usual and the ECU warning light stays off go drive for a few minutes to see if everything's ok.
If this is the case put everything back into place and you're all done !

If the ECU warning light starts to blink or the car acts funny IMMEDIATELY shut the car off and remove the cable from the ECU.
Open the ECU again and double-check for bad solder-contacts or shorts.