I already succeeded on making it a solid. Now struggling to join the two solids....
Join two solids using the combine tool.
In your case, however, I would consider keeping it as a surface body, and not making it a solid. You can use replace face to replace the old surfaces with the new surfaces:
Replacing Multiple Sets of Faces
- Click Replace Face on the Surfaces toolbar, or click Insert > Face > Replace.
- In the PropertyManager, under Replace Parameters:
- Select the first set of faces to replace for Target faces .In this example, select the five faces along the top of the "H", including the vertical faces.
- Select the second set of faces to replace for Target faces .In this example, select the five faces along the bottom of the "H".
- Select the replacement surface for the first group of target faces for Replacement surfaces , then select the replacement surface for the second group of target faces for Replacement surfaces .You must select the replacement surfaces in the order that they replace the target faces.
- Click OK .The faces are replaced and the adjacent faces of the original body are trimmed and extended to fit. The new faces are trimmed to fit the adjacent faces of the original body.