How do I update to MacOs Sierra?

What model of Mac do you have exactly?

Identify your MacBook - Apple Support

Identify your MacBook Pro - Apple Support

How to download macOS High Sierra - Can Your Mac use High Sierra - Apple Support

• MacBook models from late 2009 or later
• MacBook Pro models from mid 2010 or later

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times, a MacBook Pro battery, and a hard drive from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I had 6GB installed on an early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing

Check to make sure your applications are compatible.

Application Compatibility

El Capitan 10.11 Compatibility information

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Recommended sources of RAM for Macs are Apple Mac Memory/ RAM Upgrades from MacSales.com and RAM for Macs from Crucial.com

See here for how to install RAM on your Mac model:

MacBook Pro: How to remove or install memory - Apple Support

Install memory in an iMac - Apple Support

Mac mini: How to remove or install memory - Apple Support

You need to download El Capitan or Sierra first. If both of them give you the same message, you can’t.

Regardless of whether or not you’re able to upgrade, you should install more RAM.

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Don't even try running any of these OS versions with only 2GB of RAM. It just is not enough.

If you go to the Other World Computing web site I posted a link for, they have videos that show how to install more RAM.