you already can silly
So I'm in this base and upgrading my spinner who sweeps the floor. He is only on the screen for one of every five seconds. So I sit there and try to click on him as he passes by......nothing, nothing. Then I hype click as he passes by.....nothing, nothing. So I start tapping with eight fingers as he passes by and.......yep, nothing again. After five minutes I finally tap him and find out o am short by just a few gold's.
Needless to say, my phone has a new crack in the screen. Took me over ten minutes just to tap on this upgrade. This is not my idea of fun.
My suggestion is to simply allow us to start a upgrade from the edit screen because the traps are stationary.
Go to edit mode and long press the trap you want to upgrade.
After that, click on the damage indicators and then you can upgrade from there