How to upgrade RAM in my Mac Mini?

What year model is your Mac Mini?

see > https://support.apple.com/en-us/HT201894

For a list of upgradable Mac Mini models,

see > https://support.apple.com/en-us/HT205041

As this varies between models as to whether it's even possible, and you haven't identified your model, it's not possibvle to answer this question.

If you have the latest model it can't be upgraded. Some older models can be: you can look up your model here:

https://everymac.com/systems/apple/mac_mini/index-macmini.html

and look for RAM in the specs.