Can I replace a 6.3A fuse with an 8A fuse
A good quality adapter with no moving parts would take a full 16 amps. This  one probably has some thin wires inside to allow the various options, so they've included a fuse.

If you're charging your phone or laptop, you'll never need to change the fuse. If it's for a hairdryer, consider a fixed pair adapter with a higher rating.

In all cases, you shouldn't install a fuse with a higher rating than is specified. If it says 8 A Max, then it's probably OK with an 8 A fuse. More might risk the adapter melting down or catching fire, with a just-too-much overload.