Upgrade OS X El Capitan to Big Sur?

In order to upgrade to macOS Big Sur you need to have at least 45.5GBs of free space.

So you are going to offload a lot of files and folders you do not need on a daily basis to an external disk.

Open your Safari browser and click on the links below.

Please read this article, How to upgrade to macOS Big Sur - Apple Support

Although it is possible to upgrade directly to Big Sur I would recommend upgrading

to macOS High Sierra first and then Big Sur.

To upgrade to High Sierra click here, How to get old versions of macOS – Apple Support

go to Download OS and click on macOS High Sierra.

This will direct you to the App Store High Sierra download page it should say Get or Download.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are providing 64bit updates or alternatives.

Below are OS compatibilities for 2015 11" MBAir (base processor). This may or not apply to you but the first paragraph lists OS versions with full support for your computer, including HEVC, some type of video acceleration. The second paragraph lists OS versions that are compatible and extend to Big Sur.

I run various old Macs limited to their "terminal OS versions": El Capitan (2008 Mac Pro, 2011 iMac) and High Sierra (2012 min). Although upgrading OS can be a project, I recommend doing so as long as your hardware allows. I find that iCloud services such as Handoff/Continuity, Contacts and iCloud Drive don't fully sync to the old computers; Mail on High Sierra is unstable (quits often; so far it has never lost anything, fingers crossed).

I can not stress enough the advantage that having a second Mac OS computer makes for upgrading. Having local backup, persistent access to the internet and Apple Support let's you confidently try out OS versions and wipe/restore if you desire.

I am new to Big Sur on a new mini; I am still getting used to the graphic interface; it seems too much like small screen iOS and wastes too much screen space and seems to beg for a huge monitor. I am hoping there are tricks to learn to use screen space more efficiently, but at present I can not imagine running it on an 11" screen. I am shopping for a wide-screen monitor; you may see my topic on that in the Logic Pro discussions. I am hoping for buying advice for that question.

So my suggestion is to get that second computer that fits your work needs (larger screen?). Once that is running, you can decide how far to upgrade your MBAir.

Source: https://everymac.com/systems/apple/macbook-air/specs/macbook-air-core-i5-1.6-11-early-2015-specs.html

This system fully supports the last version of OS X 10.10 "Yosemite" as well as OS X 10.11 "El Capitan." It is fully supported by the last version of macOS Sierra (10.12) and macOS High Sierra (10.13), also. Running macOS High Sierra (10.13), it supports HEVC (High-Efficiency Video Coding), but it does not support hardware accelerated HEVC.

This model is compatible with the final version of macOS Mojave (10.14), the final version of macOS Catalina (10.15), and the current version of macOS Big Sur (macOS 11), as well.