What is the steps to migrate this?

You'll need to follow these steps:

1. Deploy a Replica Set, you can do this manually or use a template (like ) [note: i havent tested or proofed this template, this is just an example]
2. mongodump your existing mongodb database data somewhere
3. mongorestore your mongodump data to the primary node of the replica set

Notes:

• Make sure you trust the replica-set template you're using
• I'd recommend not deleting your existing service or making changes to it until after you've confirmed the replica-set migration worked. If something goes wrong you'll want something to fallback to
• I'd recommend keeping the mongodump data on your PC for a while, just to be safe - if something goes wrong you'll still have your data
• You can get mongodump and mongorestore from MongoDB CLI DB Tools

I'm going to bed now but I'll check back here when I wake up