During Varshitap you should not masturbate.

Reason in simple Jain terms:

• Varshitap is a very strong tapas (austerity). Along with food discipline, you are expected to increase samyam (self‑control) and brahmacharya (celibacy / sexual restraint). ( jainknowledge.com)
• Masturbation is counted in Jain Dharma as a breach of brahmacharya for laypeople and is totally forbidden for monks/nuns. It excites passions (kashayas), disturbs peace of mind, and brings new karmic bondage. ( jainknowledge.com)
• Any serious tapa (Upvas, Aymbil, Oli, Poshadh, Varshitap, etc.) is meant to calm the senses, not stimulate them. Sexual activity (including self‑stimulation) goes directly against the purpose of tapasya. ( jainknowledge.com)

So, if you are doing Varshitap, the ideal Jain way is:

• No sex and no masturbation.
• Guard thoughts, eyes, and mind from sexual fantasies as much as you can.
• If a mistake happens, do alochana–pratikraman–pratyakhyan the same day and recommit to your vow. ( jainknowledge.com)

You can read related guidance here and here .