Watering the Lulav, Hadassim and Aravos:
It is a Mitzvah to keep the Lulav, Hadasim and Aravos fresh and good looking. One is to do so by placing them in a vase of water. It is a Mitzvah to change the waters during Chol Hamoed in order to ensure their freshness.
Watering the Lulav, Hadassim, Aaravos on Yom Tov:
Placing the Lulav into a vase of water: The Lulav and Hadasim/Aravos may be entered into a bucket or vase of water on Yom Tov, if the water was placed into the vase or bucket from before Yom Tov. It is however forbidden to place water in a vase or bucket on Yom Tov for the purpose of placing the Lulav, Hadassim or Aravos in it.
Switching the waters: It is forbidden to switch the waters of the vase/bucket in all cases, even on Yom Tov. 
May one sprinkle water onto his Lulav, Hadasim Aravos?
This matter requires further analyses.
 M”B 654/4
 Michaber ibid
 Meaning not only may they be returned to a vase if they were removed from it on Yom Tov, but one may even initially place the plant into the vase on Yom Tov for the first time if it contained water from before Yom Tov. [so is understood from Admur ibid, and so understands M”B [336/54 in Shaareiy Tziyon 48] to be the opinion of Admur, and so understands also Kaf Hachaim 336/75 to be the opinion of Admur. As rules Admur so rules also Peri Megadim A”A 336/13; Bikureiy Yaakov 654/2; and so leans the M”B in Shaareiy Tziyon to be lenient]
Other Opinions: Other Poskim [first opinion in M”B 336/54 in name of Chayeh Adam 11/3; Tosefes Shabbos 336/21; Kapos Temarim brought in Kaf Hachaim 336/75; 654/4 and so rules Kaf Hachaim himself] rule it is forbidden to place the branch into the vase for the first time on Shabbos and only if it was removed from the water on Shabbos may it be returned to it. Regarding placing the plant for the first time on Yom Tov it is allowed to be done as even the above Poskim were only stringent regarding Shabbos. [So rules Kaf Hachaim 654/4 and Kapos Temarim]
 So is implied from Admur ibid that it has to be placed before Yom Tov and so rules M”B 336/54 explicitly.
 Bikureiy Yaakov 654/2; Kaf Hachaim 336/79; In 336/18 Admur does not explicitly mention Yom Tov, although he also does not differentiate in this aspect.
 So is implied from Admur as stated above and so rules Kaf Hachaim 336/79.
The reason: The reason for this is because one is troubling himself to “fix” the plant and it is thus similar to the prohibition of switching waters.
In Admur 336/18 he records, “and Yom Tov” regarding the prohibition of adding water to the vase. However in Likkutei Sichos 21 p. 385 the Rebbe notes that it is a printing error and really it should read “and change the water on Yom Tov”. However to add water is permitted on Yom Tov as is explained in 654/1
 Aruch Hashulchan 654/2
 The reason: As this switching of water involves a greater trouble than simply adding water, and is hence forbidden to be done even on Yom Tov. [Kaf Hachaim 654/4 in name of Peri Megadim]
 On the one hand it is forbidden to initially place water into a vase due to Tikkun Mana, and hence here too it should seemingly be forbidden. On the other hand perhaps it only applies when placing water into a vase being it is a greater trouble than simply sprinkling it. To note that SSH”K 26 footnote 97 allows dripping water onto the Lulav and dry cloth in order to wet it, hence implying such a matter has no prohibition of Tikkun Maneh.