This is what I might do.
Note, when deciding whether or not to add one (or more creatures) to an encounter, you might stagger the encounter to get a feel for how your group is doing against the encounter as written. If they are already struggling, you might just decide to keep it as it, but if they are mopping things up in this encounter or previous encounters, you might want to bring in more baddies.
My Suggested Modifications for 5 PCs ...
Encounter 1: Add another Unarmed Goblin.
Encounter 5: Increase Giant Spider Hit Points to 20. Alternately, add another Giant Spider with 8 hit points.
Encounter 7: Increase Reef Claw Hit Points to 17. Alternately, add another Reef Claw with 6 hit points.
Encounter 8: Add a Goblin Warrior, increase King Fatmouth's Hit Points to 18.
Encounter 9: Add a Human Skeleton.
Encounter 10: As is. Black Fang has more than enough room in his belly for a fifth meal!
Good luck and above all else have fun.
In general I would argue to add more monsters rather than more hit points. It will make things seem more dangerous and hectic, which is what battle should be.
Remember that as the GM, you are the only one who actually knows the numbers for monsters. If things are going very badly, you can reduce the hit points on a monster. Conversely, if things are going very easily, you can increase hit points or have another monster show up in the middle of the fight.
I would recommend not messing with things like to-hit, damage, or AC after the fight started. If a player hits AC 15 and then next around a 15 misses, everyone's going to be confused. Hit points though, they are the GM's best friend.