Most People Should Wait for the OTA
The Galaxy A Android Pie update is rolling out slowly and it will take months to complete.
If you don't want to wait for your Android update you can manually install the software right now if you have the correct device and some free time.
If you're feeling impatient, you might be tempted to go this route but you should only go through with it if you feel comfortable with the installation process.
If you feel uncomfortable with any part of the process you should hang back and wait for the update to roll out Over-the-Air.
Manually installing software can also cause major issues (it's possible to brick your phone) so if you aren't feeling confident, take a rain check.