Not updating my 15.3 iOS version

As shown in your screenshot.

You need more space on the iPhone.

Also:

You need a Wifi connection.

You need to charge the battery and preferably leave it connected to the charge circuit during the update process.

You need to turn off the Low Power mode option.

For the disk space, see if anything here helps. 

Disk space - How to check the storage on your iPhone, iPad, and iPod touch.    *

 Disk Space – Free Up    -

 Disk Space – Free Up (2).  -

 Disk Space – Free Up (3).  -

 Disk Space - Free Up (4)    -    

 Disk Space - Free Up (5)   -  

ASC YouTube for how to free up space on your iPhone, iPad, or iPod touch