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.
Exothermic and endothermic reactions both cause some type of energy level differences. In an exothermic reaction, energy is released in the form of heat. As chemical bonds are formed, energy is released. An example is water turning into ice. Chemical bonds form and release energy to create ice crystals. An endothermic reaction, however, is the opposite. Here, the energy is absorbed in the form of heat. When a chemical bond is broken, the energy used creates an endothermic reaction. Changing ice into water, then vapor, requires the chemical bonds to be broken.