You DO NOT.
Scopes do not get dirty on the inside. They are sealed at the factory and normally filled with inert gas. When you think you see dirt on the inside one of two things have normally happened. Both require repair at a repair facility not at home.
One is the seal is broken and moisture has gotten inside. The other far more common with old Weaver scopes is that the duplet lens has come apart. The glue used on the older ones tends to delaminate if you will and looks like it's dirty but what has happened is that what once was a single lens made of two pieces of glass glued together is now two separate lens.
When either happens you must let someone with the proper tools and equipment take care of it or just toss it in the trash if you're not willing to spend the money. It is NOT a home job.
