Use read -p command to save response and overwrite a config file in script [duplicate]

So I have a configuration file that I want to change after receiving a prompted response from the user.

read -p 'Your RPC Username: ' RPC_USER
sleep 1s
echo -e "${YELLOW}"
echo "$RPC_USER"
echo "----------------"
echo "Is this correct?"
echo -e "${RED}"
select yn in "Yes""No"; do
    case $yn in
        Yes ) break;;
        No ) read -p 'New RPC Username: ' RPC_USER
            echo "$RPC_USER"
    esac
done

After this I want to change a file configuration say: test.conf. So I try:

sed -i '1i RPC_USERNAME="$RPC_USER"' ~/test.conf

But only thing I get is:

RPC_USERNAME="$RPC_USER"

Again this is to be able to paste these user input variables directly into the configuration file for them during the script.

Any help is greatly appreciated!