IMAC OS update.

As Niel stated, the mid 2011 iMacs cannot be updated to Mojave. However, you can update to High Sierra, which is still supported by Apple unlike your current version of Yosemite.

See below:

https://support.apple.com/en-us/HT208969

1. No.

2. The RAM speed identifies it as a 2011 model. Mojave requires a Metal-capable graphics card, which wasn't included in pre-2012 Macs.

(173810)

Yeah,

The highest macOS you can install on your iMac model is macOS High Sierra.

Still supported, but would be the end of the road for Mac OS updates for your iMac.