Lisa, if you have additional information about an existing profile, approach the profile manager and suggest either he or you add the information. This isn't merging it's improving.
If you find that you have a profile which is nearly identical to another profile manager's (PM's) profile, You can propose a merge, (lower left on any profile, merge to lower numbers) or ask the PM if he would like to merge the two profiles. You would have to approve that your profile could be merged.
Alternately, You could propose the merge, wait 30 days (f the other PM hasn't approved it, (30 days gets an automatic approval) and do the merge yourself. Don't forget to physically combine the data in the Biography and sources after the merge, that's as important as the rest.
While many of us routinely do merges, it isn't something you should expect to have to do until you've got a fair amount of data entry under your belt.