There might be a slightly quicker way.
I presume you mean you want to remove the text strings accepted by FTM in the death field, like "infant", "dead", "deceased", etc.
If so, you can get them to group together in your index.
At the Family tab of the People Workspace, change the info to be presented in the index to be the death field. This is done at the icon at the top of the index with 3 vertical bars.
Now, go to the "sort" line, click on the down arrow, and choose to sort based on death date.
All text strings should sort between deaths with numerical dates entered (on top) and blank death dates (on the bottom).
You can now select each person who has text in the date field, one after the other, and edit the death date field as you wish in the far right panel.
Query: If you have gone to the trouble of entering that people who have died with a text string, but don't know the date, why would you want to remove the text string?