Carla, you can't edit anything from Tableau. You can create a new column in Tableau's representation of the data that does what you want, though.
Create a calculated field called Source Name. In that calculation try something like this:
IF [Source Field]='P'
ELSEIF [Source Field]='S'
Replace [Source Field] with the actual name in your data set.
Now use the calculated field instead of the raw column from your Excel file.
You can also create aliases for any dimension value. To do so, right-click on a dimension in the left menu and select "Aliases..." You'll see a menu where you can assign new values. This doesn't write back to the data source.
I prefer Michael's answer because I find it easier to understand if I'm returning to a workbook I haven't touched in a long time (or handing it off to someone else).
Thank you for the help
Thank you for this too. I was able to test out both & each were great options.
If the column is always just "P" or "S" you can right-click on the field in your dimension pane and click "Aliases"
Then on the aliases screen, under the Value (Alias) column you can change "P" to "Primary" and "S" to "Secondary". Then when you use that field in your viz it will always show as Primary or Secondary.