You want the `-e` and `-i` options to `read`:

 #!/bin/sh
 echo "@@@@@@@@@@
 @ Enter the new plarform name [for example: our-platform-7.26-2.17-res4]:
 @@@@@@@@@@"
 read -i "our-platform-7.26-2.17-res4" -e SuggestDefaultPlatform
 
 echo "you chose $SuggestDefaultPlatform"
 
From `help read`:

 -i text	Use TEXT as the initial text for Readline
 -e		use Readline to obtain the line in an interactive shell

So, the `-i` sets the default value and the `-e` allows you to enter another or modify it. You don't need the extra `echo`.