It may be possible to extend your stay as a visitor, depending on how long your initial visa was valid. However, generally speaking it is not possible to switch into another immigration category if you arrive in the UK as a visitor. You should leave the UK and make an application from overseas to return to the UK under the correct immigration category.
In some exceptional circumstances, it may be possible to apply from within the UK. For example, concessions have been introduced to allow for applications to be made from within the UK where, because of the coronavirus, it is not possible to leave the UK and make the application from overseas.