can iMac with macOS Mojave version 10.14.6 be upgraded if i increased the RAM or nothing else can be done

I don't believe there were any 31.5" iMacs, or any with 1920x1200 pixel screens. All iMacs with 1920x1200 pixel screens are so old that they are limited to El Capitan at best.

There were many iMacs with 21.5" screens (Retina and non-Retina). What is the hardware model year of yours? That will determine how far you can upgrade it. The model year should be in  > About This Mac.

• 2012 – 2013 : Can run Catalina, but nothing higher
• 2014 : Can run Big Sur, but nothing higher
• 2015 : Can run Monterey, but nothing higher
• 2017 : Can run Ventura, but nothing higher
• 2019 : Can run Sonoma

To get old versions of macOS, see: How to download and install macOS - Apple Support

If you upgrade to Catalina or later,

• 32-bit applications will stop working
• iTunes will go away, replaced by separate applications: What happened to iTunes? - Apple Support

What iMac sub-model?

From "About this Mac..."

It can be upgraded to Catalina as is. Otherwise, not if it can't already.

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No such thing as a 31.5” iMac, did you mean 21.5”? We need the model year too, please look in “About This Mac”