up ( especially BrE )
1.
to fill a container that already has some liquid in it with more liquid :
Top the car up with oil before you set off.
Top the oil up before you set off.
2.
to increase the amount of sth to the level you want or need :
She relies on tips to top up her wages.
( BrE )
I need to top up my mobile phone (= pay more money so you can make more calls) .