Leveling up in 5th Edition
So you've gained some experience and you're going to go up a level. You are now going to have to edit your character sheet and make some decisions based on how you want your character to proceed. Grab your Player's Handbook and Xanathar's Guide to Everything and dive into the process.
My method is to just work your way down the character sheet changing whatever needs changing (this is why we don't fill it out in pen). I always follow the same pattern to make sure I don't miss anything I need to change and some stats may affect others so this just reduces the amount of times you have to rub out and fill in. My method is as follows:
Class Level
Base Stats
Proficiency Bonus
Saving Throws and Skills
Armour Class/Initiative/Speed
Hit Points and Hit Dice
Class/Racial Features
Spell DC and Attack Bonus
Spell Slots
Spells
Increase your class level
Increase any base stats that need increasing. This only occurs at certain levels (4,8,12,16 and 19) and is an increase of one to two stats or 2 to one stat. Increasing your prime statistic is recommended here but not compulsory.
Increase your proficiency score at levels 5,9,13 and 17.
If you have increased stats in part 2 or 3 or you have gained a new proficient skill then increase your skills and saving throws.
This section will need updating if you increased your dexterity in section 2, if you are a bard and gain Jack of All Trades or are a monk.
Each level your hit dice increases by 1.
Check if you gain any new class/racial skills and add them here.
If your Spell Casting modifier (Intelligence for a Wizard/Charisma for a Bard etc) or your proficiency bonus increases then edit this section.
Increase your spell slots inline with the class table.
Pick your new spells if applicable.
There are a couple of schools of thought on leveling up your character; some like to map out their character from Level 1 to Level 20 making all their choices at the beginning and working out the most powerful steps to maximising their character. Others either work out their next level only or decide as they level up based on their experiences leading up to the tipping point. Both are fine but I tend towards the latter because I prefer the Roleplaying side of the game more than being an optimised combat machine. The only caveat I would make is that if you are a magic user read ahead and have a pool of spells you would like to have because there is nothing worse than leveling up during a session and needing to read a bunch of spells while everyone is waiting for you!
If you find yourself in a situation where you have to level up multiple times through a plot device or Deck of Many Things; or if you are starting your campaign at a higher level it is wise to do it one level at a time. You don't want to miss anything so just take the time to get it right.
May you roll well!