When you have two places you want to merge together (perhaps different spellings of the same place), highlight the place you want to keep in the Places page, then select Tools > Merge places from the menu.

You can then check any or all of the places you want to merge with it, and click OK. RootsMagic will then merge the selected places into the first place you chose to keep. All facts using the merged places will be updated to use the newly merged place.
Check the effects before merging
Back up the tree and inspect the events using each candidate place. Similar names in different counties or countries must remain distinct. Select the place whose name you want the affected events to use as the primary place.
After combining places, check a few facts that use the place. Check their place details, map locations, and notes. If one name is historical and another is modern, decide which name best describes where the event happened before combining them.