We have a FAQ for novices doing receptacles.
There are many tricks and traps when changing receptacles. I wrote a Q&A specifically for folks like you, please review it.
The #1 thing I'd say that isn't mentioned there is don't downgrade an outlet from GFCI to USB (unless it's redundant).
Speaking of testing, those 3-lamp testers have 3 Rather Useful lights which let you do three tests at once. (Obviously, if any lights light, the outlet is most likely hot; but that is not enough.) But the little chart/legend is completely wrong; it's about as useful as a "magic 8-ball". It's actually optimized for new construction; once built, houses have different kinds of problems.
- However, the GFCI tester function is very nice; if you suspect a redundant GFCI, use the 3-lamp tester's TEST button. It will trip this GFCI (obviously). But if this one turns stone dead and refuses to reset, it tripped another GFCI somewhere else, which means this one is redundant and doesn't need to be a GFCI.
As far as safety, once you're opening things up, turn the entire house off and check it by plugging a lamp into both outlets. There are so many surprises and gotchas for the novice (MWBC, borrowed neutrals, you name it) that it's not worth fooling around any other way. It's what I do myself in our factory building; I pull the main 480V which shuts off all the 120/240V transformers.