Trying to update the Macbook Air OS to Mojave but the progress bar freezing for about 6 hours.

amer100 wrote:

How can I fix the above issue?

Force power off your MacBook Air then restart. Sometimes the install will continue. If not, power off, restart in Recovery Mode and try and reinstall from there.

About macOS Recovery - Apple Support

That may be an indication your system does not meet requirements.

Is it backed up?

What model MacBook Air do you have, i.e. model identifier?

To my knowledge Mojave won't download if your computer wasn't compatible.

We still need to know if it meets requirements? Old Toad shows how to lookup that information.