[815C]

In Sibelius, do the following:

House Style

Edit Instruments

Select an existing guitar tab instrument (e.g. Acoustic Guitar Standard Tuning Tab)

Create New Instrument (it will ask you to confirm you want to create a new instrument based on Acoustic Guitar Standard Tuning Tab)

Edit Staff Type

You will then see all 6 strings with a MIDI note value (e.g. C2) for each string. Click on the note value and you will be able to change it to whatever you want.

After changing the notes on all 6 strings to your tuning, give it a name a save it.