Once you've logged in to Tech Log, you will be faced with a series of tiles consisting of your fleet.
Tapping on one of these aircraft will take you to the aircraft summary page after a loading sequence. Note that you must have a data connection as this loading sequence is necessary to download all the data required for your aircraft. You will then be able to access the aircraft offline.
From this page, if you wish to change aircraft, you need to tap the 3 white lines in the top right corner of the screen.
Doing so will allow you to choose to either change aircraft, or log out. Both options will return you to the aircraft selection screen but logging out will require you to re-enter your login details first. Again, to view and select a new aircraft will require a current data connection.
This video shows you how to load an aircraft from your fleet page on Tech Log and how to switch to another aircraft from your fleet.