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In D&D5, players have the choice of rolling their Hit Dice or using a fixed average value instead:
Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up).
If I roll poorly for the Hit Die, can I ignore the roll and use the fixed value instead?
No, it's either-or. Otherwise, the rule would say that you always roll and then take whichever is best. As they are instead alternatives to each other, you have to pick first. This is like all other rolls that provide for taking the average as an alternative, such as monster damage rolls.
(Mathematically, the fixed value is a better choice because it's a rounded-up average of the roll, but it offers no "gambler's thrill" or old-school methodology, either or both of which might matter more to an given player.)
That said, this is ripe for a houseruling! Make a proposal to the DM, if experience shows that the usual rule results in PCs too fragile for their campaign. (The usual advice for houseruling a new and unfamiliar system applies: play it first to see whether one's guesses are correct or not first! Often, preemptive houserules "fix" problems that only exist on paper, not in play.)
It is the tables decision, both yes and no are RAW
I have played in many games and in each game when we have reached level 2 this question comes up. It's always phrased as 'How do you want to do hp gains?' Because it's clear that it needs to be a table decision. One of the many 'rules modules' in d&d 5e.
The rule can be read both ways. Either Alternatively can either mean that this is a rule Variant and thus one replaces the other, or it can mean that you decide which method you want to use. Deciding which method you want to use can be done either before or after the roll, as there is nothing that says you can't. Depending on how you read this, will give you different answers to your question.
The important thing for the game though is that which ever way you choose to read the rule, should be agreed upon by the table and consistently used.
Some tables will only allow the dice roll. (Reading Alternative as a rule Variant decided by the DM)
Some tables will only allow taking the average (Reading Alternative as a rule Variant decided by the DM).
Some Tables will allow the player to choose at each level whether to roll or take the average (Reading Alternative as a "rule variant" decided by the Player)
Some tables will allow you to roll and if you don't like it, take the average. (Reading Alternative as an option and part of the same rule, decided by the Player)
All four are RAW. But the important thing is to remain consistent.
If your table plays with option 1 or 3 then no you can't take the average after rolling. If you take option 4 you can. (if you take option 2 there is no roll)
