I presume you are on PAYG and are opted into a Freebee package.
Freebees are valid for 30 days and you cannot add an additional pack until the current one has expired.
If you would like to switch to Talk Freebee, you would have opt out of the Freebee you are currently subscribed to and opt into the Freebee you would like to switch to.
You are only able to have one Freebee for the 30 day duration.
There is the option of a web pack with a limited of 250 mb for £5 per month. This is also valid for 30 days and you cannot add an additional pack until the 30 day expiry.
You will be able to find all the information on Freebees, with a Freebee comparison chart from the website.