Answer

This verse is not meant for touching just one single part of the idol. It is the closing verse of the Navāṅgī Pūjā, so it refers to worship of the nine limbs/parts of the Jina idol as a whole. In the traditional sequence, those nine parts are: toes/feet, knees, hands or forearms, shoulders, head, forehead, neck, heart, and navel. ( jainqq.org)

Explanation

The line “उपदेशक नव तत्त्व ना, तिणे नव अंग जिणंद” explains the spiritual reason for Navāṅgī Pūjā: the Jina, as teacher of the nine tattvas, is worshipped through nine parts of the idol. Traditional Jain explanations of this pūjā describe the worship in that sequence, and the same verse is given as the reason/summary for doing the nine-part worship, not as a command to touch only one specific spot. ( jainqq.org)

Spiritual Understanding

Spiritually, the focus is on complete reverence and भाव-पूजा. If your temple follows the common Navāṅgī Pūjā method, you touch or mark the nine prescribed parts in order rather than treating this verse as belonging to just one part. The verse is best understood as honoring the entire idol as the embodiment of the nine tattvas teaching. ( jainqq.org)

Takeaway

So, the short answer is: no single part. This verse belongs to the whole Navāṅgī Pūjā, though in the usual order the worship moves through the nine parts of the idol, ending with the navel section before the concluding verse. ( jainqq.org)