OS upgrade advice on an iMac 27-inch Mid 2010

[TheDif]

I upgraded the OS on my Mac about a year ago and bricked it. I'd like to upgrade the OS but I don't want to brick it again.

I'd love your advice please.

 

[r6mile]

Sorry, what do you mean? I had this very same machine until very recently, and it ran High Sierra very well. You might want to do a clean install. Just create a High Sierra bootable installer on an USB stick (instructions on the Apple website), erase your current partition (assuming you have a backup), and install the OS. Then, use Migration Assistant to copy your data over again. Job done!