I want to upgrade my imac memory from 8 to 16 GB

Hello,

Depending on which iMac you own, that may be possible:

Install memory in an iMac - Apple Support.

However, it's important to ask why you'd like to upgrade the Memory. If it is because the iMac feels slow in day-to-day use, that isn't a Memory issue necessarily. Figuring this out would require more investigation. The easiest way is by creating an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.

2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities.

3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.

4. Run the report, and put it in a post, as shown here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

Without knowing what iMac you own, that question is impossible to answer. Please read Writing an effective Apple Support Communities Question and then please re-write your question.