If you literally want to just convert the Full job to an Incremental, then you can simply find the job in the Job Setup window, open it, and under Settings, General, change the Backup method from Full to Incremental.
In reality you probably want to do a full backup every so often (say once a week or once a month), and do incrementals or differentials the rest of the time. To do this you'd need two jobs - one doing Fulls every so often and one doing Incrementals or Differentials the rest of the time. This could easily be done using a policy.
