RFP questions are only editable when:
Most RFPs issued via SupplierSelect require formal acceptance, i.e. the respondent is expected to review the RFP and then explicitly indicate their intention to respond (or to explicitly decline the invitation).
To accept an invitation it is necessary to change the status from Opportunity to Accepted. To do this, open the RFP and look for a link "Status" on the left hand side of the page, in the tree menu. Click this link to change the status.