दिवो द्यावा

Operational rule vidhi

For दिव् is substituted द्यावा in a द्वन्द समास of names of देवता-s.

Word by word (padaccheda)

Target दिवः ṣaṣṭhī · ekavacanaSubstitute / term द्यावा prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

दिवः द्यावा

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

दिवः → द्यावा

Replace divaḥ with dyāvā. Inherited words (anuvṛtti) may add further conditions.

For दिव् is substituted द्यावा in a द्वन्द समास of names of देवता-s.
Thus द्यावाक्षामा, द्यावाभूमी ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.