First of all, I hope you already have source control (Azure DevOps) set up. If not, please start by doing it.
Next, you need to have two branches in ADO - for example DEV and MAIN.
You can build&deploy code from DEV branch to your "dev-test" system. Once they are tested, you can merge those changesets to MAIN branch, and build&deploy the MAIN branch code to your UAT system. Finally, once you've happy with the UAT version you will install the UAT package to Prod.
This topic (dev ALM) is quite big, so I suggest you to try to search for documentation, discussions and blogs to gain some understanding. If you have specific questions after that, you can post new questions on this forum.