Multiple ways to Convert tabs with spaces in Notepad++

In order to convert tabs with spaces in Notepad++, go to Menu: Edit -> Blank Operations and select TAB to Spaces

If you want to convert each tab in the file with single spaces, you can make use of the Find and Replace option with Search Mode Extended as follows,

1. Open File in Notepad++
2. Press Control + F to Open Find and move to Replace Tab,
3. Make sure to select Search Mode as Extended,
4. Add, Find What: \t
5. Add, Replace With:    (a single space, or multiple if you want it that way!)
6. Click on Replac All

If you want to replace TAB by space (4 spaces or more) as default permanent behavior, then you can do the changes in Preferences as follows,

1. Go to Menu: Settings,
2. Now select Preferences...,
3. Select the Language tab,
4. Now under tab settings check the Replace by space checkbox.