It is going to depend on what version of Android your phone is running. If it is anything older than "Marshmallow" then you won't be able to do that. Also, if the manufacturer has restricted the option (sadly they do that some times) then you wouldn't be able to do it either.
That said, here are the instructions on... How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now...