What if you create a calculated field to replace Region with code like this:
IF [Country] = "India" THEN "IMEA" ELSE [Region] END
That will give you IMEA as the region for India and use the existing value of Region for all other countries. You could then remove the old Region field.
Hope that helps!
Peter, the concept here is that Tableau can't change what's in the column. The data is the data. But Tableau CAN create a brand new column to hold the values you want. You just create a calculated field as Joshua suggests. Call it "Region (Corrected)" or something like that. Use that new column wherever you would have used the original "Region" column.
I wish Tableau's documentation explained concepts like this better: this is an extremely common question for new users and the answer, while simple, is not obvious!