I usually wear a long-sleeve shirt but I fold up the sleeves up to arms due to the hot weather all the time these days. I pray while my sleeves are folded up. Is Haram or Makrooh to fold up my sleeves? I fold my sleeves all the times, not only during the prayers.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

al-Bukhari and Muslim reported from Ibn Abbas that the Prophet (Sallallahu Alaihi wa Sallam) said: The Prophet said, "We have been ordered to prostrate on seven bones and not to tuck up the clothes or hair" .
al-Manawi said: 'do not fold the clothes or assemble them while making Ruku'h (bowing down) or prostrating in the prayer'.
al-Nawawi mentioned that folding the sleeves is part of this'. al-Nawawi also mentioned that there is a consensus among all the Muslim scholars that this is just dislikable not prohibited.
If the person folds his sleeves during the prayer, there is a consensus among the scholars that this is dislikable. But if the cloth is folded before one starts praying, there is a disagreement among the scholars over this issue. Some believe that this also is dislikable while others do not think so. The majority of Muslim scholars believe that such an action is dislikable.
But it is more appropriate for the person who is going to start praying not to fold his sleeves.
Allah knows best.