how to clear previous value in gridview using c#

Question

 In Gridview as follows
 
Room    course       Room course
 

11      LGTF                21    PH1
Edit(link button)   Edit(link button)
 
12 RM                        22     ERS
Edit(link button)     Edit(link button)
 

13 REO                       23     PH2
Edit(link button)      Edit(link button)
 

when i click the edit link button in pop up the course will be shown in dropdownlist.
 
if the user want to change the course for particular room, click the edit(link button) and select the course in the dropdownlist and click the ok. in the gridview changes course will be shown in gridview.
 

for example, for room 11 LGTF is there,if the user want to change the course From LGTF to PH1 means, in the room 21 already course PH1 is alloted, that room 21 course PH1 is to be cleared in the gridview.
 

for that how can i do in asp.net using csharp.
 
Rgds,
Rao