How to update my iMac to the latest macOS?

Hi. I.m trying to update my iMac to the latest OS.

Its a 21.5" late 2013, 2,7G quad core i5 with 8gb ram.

I recall a while ago, i got a box on startup to upgrade to big sur. I closed the window and forgot about it.

Since then due to home improvements, the mac was packed away and we used phones and iPads etc.

Now got it set up again, but everywhere I turn now all i get is update not available.

Is my mac not upto spec? I can't see much difference in spec between my mac and a 2014 mac?

I was trying to update my Garmi satnav, but the existing app wouldn't work and the new app will only work on big sur.

Any advice would be most appreciated.

Certainly not in a position to warrant spending the money, to get this app and others to work should this occur , on a new mac.

Many thanks

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