Log in to Astroprint and launch the Monitor.
The monitor will show all 3D printers that are connected to Astroprint. If the printer you will use is already connected, launch the Camera/Controls. If it is disconnected, make sure the printer is on before proceeding.
Under the "Printer Controls" tab, click the pencil icon above the "Nozzle" temperature gauge and enter a target temperature (180 for PLA). The hotend temperature indicator should begin to rise and slowly approach the new set temperature.
Once the set temperature has been reached you can slowly pull the previous filament out of the hotend and feed the new filament in.
TURN OFF the hotend by clicking "Off" under the "Nozzle" temperature gauge, unless you are going to begin a print immediately.