Current Job Title is a default system field and the field type is a dropdown. You cannot change the dropdown values manually within a specific record. You can however add a new value to the dropdown by going to Setup > Customization > Modules > Candidates > Layout > Go to "Current Job Title", hover on it and click the Gear icon > Edit Properties > hover to one of the values and a + icon will appear, click it and add the new value > Click Done > Click Save.

Then, go to the record, change the value to the newly added value.