Ran into a similar pothole yesterday when i wanted to manually input a barometer setting. Same popup, same behavior--had to clear the existing setting before I could enter the new one. In this case, it would be helpful if not only did user input immediately start to replace the current barometer setting, but it would be good to auto-insert the decimal point when the third digit is pressed. So, instead of typing "30.05", just "3005" would result in an input of 30.05. Further, if you enter "299", iFly should interpret that as "29.90".
This behavior is similar to a common input shortcut on modern avionics with keypad entry for radio frequencies, so would be familiar to most pilots. (i.e., you can punch in "1227" to tune in "122.700". In fact, many (most/all?) such devices would also recognize "227" as "122.700", as the leading 1 is assumed since all freqs begin with a 1, but that would not apply to the barometer setting case, since it could either start with a 2 or 3 so the user will need to specify that first digit.)