Have you looked through all the threads here (the Mature Study board)?
There must be countless people currently in the midst of or just about to start Master’s courses. Do read the threads already in existence for every possible tip on making return to study easier.
Is this a field you’re currently working in? If so you can feel pretty confident. If on the other hand, you’re stepping into fairly new territory, be prepared to feel a bit exposed. Even if you’re not usually shy, or retiring, the seminar / lecture room / lab (?) environment can feel quite hierarchical and daunting at first.
But it’s likely your fellow students will be delightful, welcoming, considerate and respectful. You don’t have to pretend to be 20 something, and you may find you have a different relationship with your tutors to what might have pertained when you were much younger.
Get on top of all procedures as early as possible. I have no idea how things work in the sciences but do the equivalent of familiarising yourself with physical and online library systems, submission guidelines, referencing … (This is probably all completely obvious to you - it wasn’t to me …)
One other thing - maybe less relevant in your forties than it was for me in my fifties - you will be exhausted for all of the first term. Have you been allotted a locker at your institution? I wasn’t, despite pleading, so my very long commute also involved having to carry everything I needed for a long day around with me all day. I dropped two dress sizes in two months and had to buy a whole new wardrobe.
Oh yes - exhaustion. You may not have the energy to pull all nighters - so plan your work. Split things up into small amounts and don’t leave anything to the last minute. In fact it’s best to assume you will be ill for the entire week before a deadline.
I did my undergraduate and professional study and training in the 20th century, mostly without benefit of the Internet. Absolutely the nicest thing about my much more recent MA was being able to do most of my research under the duvet, on my phone kindle.