While in Edit Mode, select the parts of your mesh you want to convert and use Alt + J or from the Faces special menu with Ctrl + F > Tris to Quads to convert tris to quads or via the 3d view header, use, Mesh > Faces > Tris to Quads.
This tool converts the selected triangles into quads by taking adjacent tris and removes the shared edge to create a quad, based on a threshold.
To create a quad, this tool needs at least two adjacent triangles. If you have an even number of selected triangles, it is also possible not to obtain only quads. In fact, this tool tries to create "squarest" quads as possible from the given triangles, which means some triangles could remain.