You can write a formula in a new cell to combine cells. For instance
if the first name is in A1 and the last name is in B1, you can write a
formula in C1 like this:
=A1&" "&B1

This joins the value of A1 with a space and the value of B1. If you
need to you can arrange it to be last name comma first name:
=B1&", "&A1