im currently hold 3 user agent then i want to upgrade to 5 user tier agent. So once i acquire the license i will just apply it and it will be 8 user tier on my service helpdesk ? am i correct?

[Nic Brough -Adaptavist-]

No.  The licences are done in tiers, they do not accumulate.  A tier represents the highest number of people you can give access to.

You have a 3 tier licence at the moment, so you can have 1, 2 or 3 agents.  The next tier is 5, which would let you have up to 5 agents.  If you want to have 8 agents, then you need to go to the next tier of 10 agents.  (When you upgrade, your existing 1-3 licence will be taken into account in terms of cost, but it is replaced by the 5 or 10 tier, not added to)