How to evolve Magneton?

To evolve Magneton to Magnezone you just need to add a magnetic lure module to a Pokestop that you are close to. Then you should be able to click the evolve button.

Basically, if you don't have a magnetic lure module, you can pick one up on the shop for 180 coins. You can get up to 50 coins a day by occupying gyms.

Then when you have a magnetic lure module, simply tap a Pokestop that you are withing range of, then tap the rectangular white icon and there you can add your magnetic lure module, it last for about 30 minutes ad as well as being essential to upgrade Magneton to Magnezone, you'll find that it also attracts Pokemon to the area, especially electric, steel and Rock type Pokemon.