Plan upgrade (Workspace) and we need more space and don´t know how to upgrade

Hi all,  We have a starter Plan (Workspace) and we need more space  and don´t know how to upgrade -I mean to contract an upper contract- .

Can anyone help me how to do it , please? 

I see no help guide about thsi aprticular case in the squarespace webpage 

Many thanks n advance  ( sorry for my clumsiness)

Rafael