Welcome, Antoine, to the OEM forum.
Yes, it is very easy. You download the sketch you want to use from Github, and you upload that to your emonTx, in exactly the same way that you uploaded the 3-phase sketch.
But, I do not know which sketch was in your emonTx. On 18 October, it was announced that EmonTxV3 Continuous Monitoring Firmware (v1.0-beta) became the default.
Before that date, the default was https://github.com/openenergymonitor/emontx3/tree/master/firmware/src
It is very likely that this is the one you had.