Gmail – Is it possible to merge two labels in Gmail

There are quite a lot of ways to solve your problem:

• You can hide the label, so you don't have to see it every time: go to Settings > Labels and choose the labels you want to hide.
• One other way would be to use nested labels. You need a root label; you can call it Old, for example. Then, rename the Amsterdam label to Old/Amsterdam. That way, the Amsterdam label will become a sublabel for Old.
• If all you want is to archive the conversations, search for label:Amsterdam, select all the conversations and archive them.

Go to Settings (the cog wheel in the upper-right corner) → Labels → click Edit for the respective label → click Nest label under and select the respective label under you wish to nest it (“Extracurricular” in your case) → click Save. Of course, you should create the “Extracurricular” label first.