Upgrade Macbook Pro to Catalina

A 2015 MacBook Pro can upgrade to all available OS's including Catalina and the newest macOS Big Sur.

although it is possible to upgrade directly to Catalina or Big Sur if you are interested I would recommend

you upgrade to macOS High Sierra 10.13 first as this will prepare your mac for the new APFS filesystem.

Read everything below thoroughly and all the information contained in the blue links also.

For macOS High Sierra and macOS Catalina,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to the Download OS section and click on the OS you want to install.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

Connect your mac to your router via cable rather than rely on WiFi.

Do not use your mac while it is downloading

and do not use any other devices that share the same internet connection.

If the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you want to or are able to install Big Sur, open Safari and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are offering 64bit updates.

You're still running a very old system. There's a few things to take in account.

First, your Mac can run up to Big Sur, the newest OS.

Catalina and newer don't run 32 bit apps, so, if you have any, check if they have 64 bit versions for the newest OSs or find a replacement.

Don't look upgrades from where you are to where you're going. There's a lot changes since El Capitan, so the best is to make a clean install.

For the clean install:

BACKUP - as I said before, go for the clean install, so the best backup in this case is to copy paste your personal data (pictures, songs, documentos, etc.) to an external drive (2 drives if you have available), so you can set everything up as new and then copy back the data. You can use Time Machine, but it copies old files that can be corrupted or incompatible and can cause you some trouble.

Now this:

How to create a bootable installer for macOS - Apple Support

With this bootable drive plugged in, restart holding Option and choose the installer to load the Recovery Mode

Once in RM, open Disk Utility and erase the drive completely How to erase an Intel-based Mac - Apple Support (skip step 1 because that's what the boot from the drive has got you to)

Disk erased, back to the main menu and select Install macOS.

Follow the process, sign with your Apple ID, etc. and it'll boot a fresh and clean macOS.

Now allow it to install any updates it asks for, install your favorite apps and copy back your data to the disk.

That's it. A bit of a work but it's the best to do and it'll get you the best performance of your Mac, free from debris like files and settings from older OS and apps that could cause you some trouble.

Take a weekend to understand and perform the process, watch some videos of the process on YouTube so you feel safer.