The current version of the +3e ROMs is v1.43. Improved versions will be posted here from time to time.
First, unscrew the case of your Spectrum. There are five screws on the bottom of the case, and two at the right-hand side on the disk drive. Remove the top by lifting at the right-hand side first, unplug the red/black cables that connect the power light to the circuit board, and take care to ensure that the keyboard cables at the left are not dislodged.
Near the edge connector at the back of the circuit board, you will see three socketed chips, one slightly larger than the others, as shown in the following diagram:
Carefully remove the two smaller chips by placing a flat-bladed screwdriver under each end in turn, and gently easing the chips out. Keep these chips safe in case you ever want to return to the original +3 ROM (note that the chip labelled 40092 is in the position marked "ROM A", and the one labelled 40093 is in the position marked "ROM B").
Now insert the replacement ROMs in the appropriate positions, taking care to ensure that they are placed the correct way round (with the semi-circular "notch" towards the front of the Spectrum) and that all the pins are properly located in the socket.
Finally, replace the cover (reconnecting the power light). Switch the Spectrum back on, and you are in possession of a ZX Spectrum +3e!
To do this, you will need to have access to an EPROM programmer. You will also need two blank 32K EPROMs, of the 27C256 variety. Actual EPROMs that you can erase and re-use are now becoming hard to source, but OTP (one-time-programmable) versions will work just as well (although you will of course need a new set when a new version of the +3e ROMs comes along).
Alternatively, you could use electrically-erasable SST 27SF256 EEPROMs, which are readily available (thanks to Zoltán Böszörményi for this tip!)
Download the following file which contains versions of the ROMs for all interfaces in both English and Spanish:The names of the ROMs contained in the zip file are encoded in the following way:
For example, if you want the English ROMs supporting the divIDE interface, suitable for blowing with an EPROM programmer, you will need the following two files: