Hi!
I had the same problem too with an old iPhone 5 16 gb.
A solution could be maximize the storage on your device following apple suggestions:
How to check the storage on your iPhone, iPad, and iPod touch
https://support.apple.com/en-us/HT201656
If you need more space for an iOS update
https://support.apple.com/en-ie/HT203097
I was personally trying to not keep a lot of applications on my device, but later I understood that nowadays as we are using a lot of social applications, iCloud services, multiple email accounts, games, etc. 16 gb are not enough anymore in order to be able to use the phone as we would like to... so I just decided to buy a new one of 128 gb!