I have an old P60 that I was thinking about doing my first MOD too. I wanted to throw an XML2 emitter in it replacing the XPE. What are the steps/tools involved?
If nothing else, a soldering iron is a must. Remove the drop-in from the light head, unscrew the reflector, desolder wires, remove existing star, clean the pill, apply thermal compound, place the new star, resolder wires, place insulating ring over the star, screw the reflector back, assemble the light. Voila!
What do you want ?
Simply an emitter swap ? or change the driver as well ?
1) Simple emitter swap , un solder wires + and - , remove emitter ( this can be a PITA ) , clean pill surface , glue in new one ( thermal glue ) , and re solder wires , being aware of where + and - are on the new emitter ... I use 2 x AA for this as it wont fry the emitter ( 3v ) , good to be sure ...
Or you can order parts , and build a new drop in ...
Tools :
Pliers , Soldering iron , something to pry out a stuck emitter ( sometimes they are seriously glued in place ) , thermal glue like Arctic Silver
Just looking at your post count and wondering if you realise the importance of the driver? 
Further to what old4570 said above, don't expect a massive improvement unless you replace the driver also. The XM-L2 will output more light but it is unlikely that the gain will be huge since the XP-E driver won't push it anwhere near hard enough to realise its potential.
Also keep in mind that in a small reflector such as found in a P60, the XM-L2 will be MUCH more floody than the XP-E.
In short, with just an emitter swap, you will gain a little in overall output but to the naked eye and over longer distances your P60 may actually appear dimmer.
For you, it would be easier to build from scratch.
The existing driver probably isn’t a good match for an XML2, and the existing reflector probably won’t fit an XML2 either. You’ll probably need to replace them both.
Between swapping out the emitter, driver, and reflector, you’re basically doing a complete rebuild anyway, so you might as well buy a new pill also and build it from scratch. It’ll be much easier than the alternative.
Instructions for DIY P60 build.
Please order a new pill for XML and begin from scratch as above mentioned.
Driver and pill will cost approximately 5$ and you will have the old one as a spare and to compare the beams…
If your p60has a xpe reflector it wouldn’t fit a XML anyway.