Replacing Some data

You can achieve this with incremental refresh. Either in:

• semantic model: https://learn.microsoft.com/en-us/power-bi/connect-data/incremental-refresh-overview
• dataflows: https://learn.microsoft.com/en-us/power-query/dataflows/incremental-refresh

You can remove old data and load the latest records using Power Query in Power BI. You need to:
1. Load existing data
2. Filter out records from the last 3 months
3. Append new data from the CSV

You can also use Incremental Refresh to automate this process.

https://learn.microsoft.com/en-us/power-bi/connect-data/incremental-refresh-overview

Let me know if you need further guidance

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Hi tonyenglish , There are several ways to achive this through Power Query as well as incrimnetal refresh please try these out 
Incremental Refresh:

• Set up incremental refresh in Power BI for your dataset.
• Configure it to delete and reload data for the past three months during each refresh.

Filter Old Data:

• In Power Query, filter records older than three months.
• Append the new data to existing data.

If your source is Database then you could use SQL to filter it in the source itself
If this post helped please do give a kudos and accept this as a solution
Thanks In Advance