NOTE: You need to have SP1 installed on Windows 7 in order to upgrade to Windows 10 using the Windows Update method shown in this video. If you are upgrading from Windows 8.1 some apps may not be supported yet on Windows 10.
You answer your own question. There is a fall in temperature because the reaction is endothermic. The reaction needs energy, so unless you can supply some heat, the temperature will fall. Eventually, some heat from the surroundings will transfer and equilibrate the temperature. Temperature of the system is a measure of the global energy. Most reactions are exothermic so now the temperature rises and then transfers to the surroundings. A very famous example is a burning fire.
If heat is given off, the reaction is exothermic. Exothermic means that during the reaction, molecules have moved from a higher state of energy to a lower state. An endothermic reaction is the opposite. This is when a reaction starts colder and ends up hotter, taking in energy from start to finish. In an endothermic reaction, the system gains heat as the surroundings cool down. In an exothermic reaction, the system loses heat as the surroundings heat up.