Remove the frequency module from the back of the transmitter.
Be VERY careful...
If necessary, use a blunt knife ( such as those you would eat with ! ) to push in the tangs that hold the module within the metal case.
Do the top first - so it sticks a little way out and you can grip it with your finger ends, then do the bottom whilst holding the top !
Then pull it straight out... STRAIGHT OUT...
There is a long multipin connector within - and if you pull the module out crookedly - you will bend the pins.
With the module completely out of the transmitter, you will be able to see the elongated oval slot in it - where the transmitter crystal goes.
___________________________________________________________________
If you push on the rear/bottom of the gold case here and there, you will discover where it seperates.
The Gold Tx that I have is a very rare 3-channel one for cars - yours is a much more common aeroplane radio - but they are all similar.
Again - Be CAREFUL - that transmitter ( and likely also the module ) are as good as irreplaceable in such good condition ! ! !
Even more so in Chile !
You may need to remove a few bolts or screws that hold the rear/bottom to the sides - mine is only clipped together !
Inside, there will be a black plastic case holding 8 rechargeable 600mah AA nicads.
The case will be bolted into the metal transmitter case with a pair of long M2 bolts.
Remove those bolts ( it takes quite a while as the thread is FINE ) and remove the battery pack.
My advice is that you dispose of that properly - and look online for a replacement.
You will be able to find an eight-cell welded pack of AA NiMH cells of about 2400mah ( 4 times the capacity ) - for maybe US$20 plus shipping.
Alternatively - there may be a good shop in Santiago ?
If you have the charger for that transmitter - and you get a battery with more capacity - please bear in mind that you will need to adjust the length of time that you charge a higher capacity battery pack.
Simply fit the new battery with double-sided tape - into the front portion of the transmitter.
I replaced the 600mah battery in my 3-ch transmitter with an eight-cell AAA NiMH 750mah battery pack - but my transmitter is intended only for cars and boats - whereas yours is meant for the control of sophisticated aircraft...!