Step 1. Sign into the Battle log
http://battlelog.battlefield.com/bf3/
Step 2. Click your Gamertag
Step 3. Select the "Dog Tags" tab
Step 4. Click "Edit Dog Tag"
Step 5. Select the dog tag you want
Step 6. Hit "Save Dog Tags"
You have successfully changed your Battlefield 3 Dog Tag!
http://battlelog.battlefield.com/bf3/
Step 2. Click your Gamertag
Step 3. Select the "Dog Tags" tab
Step 4. Click "Edit Dog Tag"
Step 5. Select the dog tag you want
Step 6. Hit "Save Dog Tags"
You have successfully changed your Battlefield 3 Dog Tag!