First, up front, there isn’t a direct “on page such-and-such it says you can dismiss psionic focus by…” answer to this question. On this point, I cannot prove a negative but I am quite confident. We have to extrapolate from existing information about what psionic focus and its expenditure is. Therefore:
The basic usage of psionic focus, assuming no relevant feats or class features, is to “take 15” on a concentration check. Concentration checks don’t usually require any kind of action—they are automatically prompted by circumstances (i.e. something trying to disrupt your concentration). This, to me, strongly suggests that there isn’t really any “effort” involved in expending psionic focus. Certainly, there are ways in which you can expend psionic focus without using any action at all, not even a free one, considering how concentration works.
Moreover, while the rules for concentration focus on disruptions during spellcasting, there isn’t really any reason why you can’t just concentrate on something else, and can’t just make a concentration check “for funsies.” Call it, ya know, “meditation”—which is a pretty big part of psionics.
Furthermore, you can change your chosen energy type by “re-focusing” without dismissing your psionic focus. You don’t focus “twice” (unless you have something that actually lets you do that, like the Deep Focus feat), but you can change stuff that you pick on focusing. This takes the same amount of effort as focusing would otherwise (so usually a full-round action, unless you have Psionic Meditation). So really, the whole question is moot. (If you do have Deep Focus, the same still applies: you can just go through your usual focusing routine to change your active energy type, even if you’re already “full” on your foci.)
Finally, a bit of editorializing: Pathfinder psionics is based on the Expanded Psionics Handbook psionics for D&D 3.5e, which is also available as open-game content, e.g. here. In 3.5e, manifesters never picked an energy type when they focused—they could just freely choose the energy type they used for each power they manifested. See the 3.5e version of energy ray for example—no mention of “active energy type,” because that wasn’t a thing, instead you just chose when you manifested the power. This change to make the active energy type fixed when you focus was a mistake. There is simply no good reason for it—it’s awkward and makes some of the weakest psionic powers weaker. I strongly recommend that your GM just remove it from your games. They’ll be better for it.
And my source for all of this is that I worked for Dreamscarred Press for a time, on psionic material. I came after Ultimate Psionics—and so missed the opportunity to push back against the active energy type—but the fact that they paid me to write psionic material for them is hopefully worthy of consideration. I was hired on the strength of my homebrew portfolio and the deep conversations I’d had with DSP devs about psionics, and those things were based upon my extensive experience with psionics as both DM/GM and as player. I have pretty considerable experience with games that skip the active energy type, either 3.5e where it never existed or Pathfinder where it was houseruled out, so that recommendation is coming from a place of pretty deep expertise.