How can metamorphic rocks be turned into sedimentary rocks?

There are three types of rocks: igneous, sedimentary and metamorphic. These rocks can be transformed into each other by long-term processes and are part of the rock cycle. 

Metamorphic rocks can be converted into sedimentary rocks by the processes of weathering and erosion. Metamorphic rocks can be broken down into smaller fragments by weathering agents such as water, ice, acid, salt, and plants. These small rock fragments can then be transported over a short or long distance by erosion. Agents such as air, water, ice, etc. can cause this transport. These transported fragments are deposited over each other and over time, a large number of such fragments are deposited. The weight of overlying fragments causes the spacing between the fragments to reduce and over time, fragments get cemented to each other, thus forming sedimentary rocks.

Such a transformation process can take millions of years. 

Hope this helps.