I need to update so that I can download keynote

The age of your Mac dictates what you can upgrade too. You will need a 2015 or later MBP to upgrade to Monterey, and one from Late-2013 or newer to upgrade to Big Sur. The current version (v11.2) of Keynote in the Mac App Store requires Big Sur or later.

You may be able to get Keynote v11.1 via the Mac App Store. Launch the Mac App Store on Catalina and be signed in with the Apple ID that you have used to download Keynote in the past. Press cmd+0 (zero) to open the Purchased panel, locate Keynote, and then click either the Update button or a cloud symbol (if present). You will receive a dialog telling you that you need a newer version of macOS. Click OK, and hope that a second dialog appears offering to install the last compatible (v11.1) version of Keynote for Catalina. If it does, then you may not need to upgrade to Big Sur or Monterey, unless there is another reason driving that decision.

Tips Big Sir Upgrade

Revision # 15 - November 25, 2021

Tips How to Upgrade to Big Sur 

There are several points to consider before attempting the upgrade to Big Sur. The most important is Due Diligence on the part of the User and is a Prerequisite for a smooth upgrade.  

The below points should all be consider Before attempting the upgrade.

Point # 1 - Are there pre-existing Unresolved Software Issues. They should be resolved prior to upgrading - they may carryover to Big Sur making the system unstable / unusable.

Point # 2 - Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The The Built in Security  is all that is required.

Point # 2 A - Commercial VPNs should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.

Point # 3 -  How to get old versions of macOS  ie: Empty Space Requirements specific to the Big Sur. Apple states 35.6 GB Empty Space but suggest 50 GB

Point # 4 - Does the intended computer qualify for the upgrade.

Point # 5 - Are the installed Applications / Extensions compatible with Big Sur. Check with the Developer for compatible Apps / extension or do they offer updates. Same goes for Third Party Drivers for Printer, Scanners etc.

Point # 6 - If upgrading from Mojave and earlier versions of macOS - Remember that Big Sur does not support anything 32 bit. These include Applications, Extensions and Drivers for any devices. There is no workaround.

Point # 6A - To check for 32 bit Applications - a good application to use is Go64 - safe to use.

Point # 7 - Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Point # 8  - To upgrade to Big Sur will requires a quick, stable and reliable Internet Connection - preferred method is via Ethernet Connection. 

If not available and using Wifi - move computer as close to the Access Point / Router and disconnect all other devices connected to Wifi. Want the maximum bandwidth for the computer being upgraded to Big Sur.

You can download Keynote from the AppStore.