I have a 2015 MacBook Air with macOS Sierra 10.12.6. Should I replace with MacOs Mojave.

The 2015 MacBook Air is capable of running the newest macOS Monterey.

If you use still heavily rely on some critical, older 32-bit apps (that will not run in Catalina, Big Sur or Monterey) then Mojave is the best upgrade path for you at this time.

❗️ – IMPORTANT – Your 32-bit apps won’t work in Catalina, Big Sur or Monterey.

https://support.apple.com/en-us/HT208436

Use the free utility Go64 to check the compatibility of your apps:

https://www.stclairsoft.com/Go64/

If you already have or are ready to update your apps to be compatible with Catalina or newer macOS, then I'd suggest you upgrade to either of those newer OS versions.

❗️ Do NOT upgrade until you have made a Time Machine or other backup of your startup drive.

Learn How to Back up your Mac with Time Machine

How to Upgrade  macOS:

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

✅ - IF your Mac and apps are compatible, then using Safari only, download the macOS upgrade installer app:

Click > Mojave,  Catalina,  Big Sur.

Once downloaded, double-click the “Install macOS…” app in the Applications folder to begin the upgrade.

EDIT - I'd still wait a little bit before upgrading to macOS Monterey. Let the dust settle.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence

Depending on the exact date of this computer it may qualify for the latest macOS Monterey

One may consider upGrading to High Sierra first if considering upgrading to any version above High Sierra. This may better position the computer for a possible  Drive Conversion to the newer APFS format as well as any EFI BOOT ROM required to use the newer file format.

Tips Big Sir Upgrade Released Nov 2020 - Empty Space requirements 35.5 GB - Suggest 50 GB

Tips Monterey Upgrade  Released Oct 25, 2021 - Empty Space Requirements 26 GB - Suggest 50 GB

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

That is a good argument, are you sure that the latest Office365 and AAP versions will run in Mojave? if so, you can upgrade to Mojave. Download and install it. Then download and install Off365 and AAP.

What would you like to have that is not in Sierra?

The questions been answered by @ D.I. Johnson and @ P. Phillips above.

Just follow the directions from either of these Contributors.