The last thing I did was write "Happy Birthday" on his cake, then I moved on to the smash cake. When I came back to do an "overall" look of the other cake, I cringed. The first thing I did was put his name on the side of the cake, then I placed colorful circles on the cake. Then I decided to write in red buttercream "Happy Birthday" on the top of the cake. Evidently I turned the cake and "Happy Birthday" doesn't line up with his name on the side of the cake. How can I fix this?

what style of cake is it

Has the buttercream crusted? You can usually just pull off the writing (gently) with minimal damage and re-pipe it in its correct spot.

The buttercream had crusted, and of course the writing was in red on top of white. I scraped it off and re-iced that portion of the cake - there were some decorations that I could not remove at this point. Re-smoothed the top, and wrote again. I've tried to upload pictures but can not do so. Thanks for your help.