Before you can update guest permissions, the person must be added as a guest to your team (see also Add guests to a team).
Guests have fewer capabilities than team members, but there's still a lot they can do in channels—where the work actually gets done in Teams! Team owners can set guest permissions for channels to control this. For more details, see Guest capabilities in Teams.
To set guest permissions for channels in Teams:
Select Teams on the left side of the app.
Go to the team name and select More options > Manage team.
Select Settings > Guest permissions. Check or uncheck the permissions you want to use. Currently, you can give guests permission to create, update, or delete channels.
Your guests can now collaborate with team members in channels.
Note: File permissions for guests reflect whatever your admin has set in your SharePoint settings. These can only be changed by your admin.
Team owners can set guest permissions, but only from the desktop or web app.