Unfortunately we do not yet have a merge feature for materials as there is a lot of complexity around recipes and manufactures.
As a solution to being able to directly merge your materials, it may be best to move across all of your expenses to the one you wish to keep. This will keep you from having to recreate these from scratch.
To do this:
1. edit each of your expenses containing the material
2. remove the line item and re-add it with the new material.
3. switch the material to the new one in any recipes that use the material
4. backdate any manufactures that contain the material by removing the manufacture completely and then re-add it with the new material.
Alternatively, you might like to keep the old materials and some data associated with them and then mark them as archived. For your new ones, you could "bring forward" any quantities by adding a one-time Inventory Adjustment to account for the current stock on hand.