To replace missing values in Power Query, follow these steps:

1. Load your data into Power Query:
- Open Excel and load your data into Power Query by selecting `Data` > `From Table/Range` (if your data is in a table) or `Get Data` > `From File` > `From Workbook` (for other Excel files).

2. Select the column with missing values:
- In the Power Query Editor, locate and select the column that contains missing values.

3. Replace missing values:
- Right-click on the column header and select `Replace Values`.
- In the dialog box that appears, leave the `Value to Find` field blank (to signify missing or null values).
- In the `Replace With` field, enter the value you want to use as a replacement.
- Click `OK`.

4. Alternative method using the Transform tab:
- With the column selected, go to the `Transform` tab on the ribbon.
- Click on `Replace Values`.
- In the dialog box, leave the `Value to Find` field blank.
- Enter the replacement value in the `Replace With` field.
- Click `OK`.

5. Alternative method using the Fill feature:
- If you want to fill missing values with the value from the cell above, you can use the `Fill` feature.
- Select the column with missing values.
- Go to the `Transform` tab on the ribbon.
- Click on `Fill` > `Down` or `Up` as needed.

6. Apply and load the changes:
- Once you have replaced the missing values, click `Close & Load` to apply the changes and load the data back into Excel.

By following these steps, you can effectively replace missing values in your data using Power Query.