How can I fix my Bauknecht freezer?

I have a Bauknecht built in freezer which is no longer freezing to an adequate temperatue although all electrics seem to be working i.e. warning beep, fast freeze button. The available opening is W56.5xH97XD55cm and all new built freezers appear to have a width of 59cm or more. Therefore I need to repair my present freezer - how do I go about this?
I would appreciate a reply asap. Many thanks. Judy Waters