From the 'Extensions' menu at the top of the page click on 'Manage'
On the left-hand side of the page click 'Update'
The center of the page will show extensions that need to be updated. In the image below you can see 'JCH Optimizer for Joomla Pro' and 'pkg_gantry5' both need updates. Click the tick box of which extension you want to update.
Click the 'Update' button up the top left of the page next to 'Find Updates' to update the extension.
Repeat steps 4 & 5 for all extensions that need updating. I recommend you only do 1 at a time, rather than trying to do them all in one go, as that can cause problems.