Step 1: make a backup of your data
1.1. Make a backup of all your files inside the htdocs-folder.
1.2. Go to phpMyAdmin, and export all your databases one by one. Export them as SQL-files. Don't forget to turn on the 'Send'-checkbox at the bottom of the export-page or else the data of the database will be displayed in a textarea-field instead it will be downloaded to a file.
1.3. If you've made changes in other folders then the htdocs-folder, such as in httpd.conf or in php.ini, remember what you had changed there.
Do step 1.2 and 1.3 of above, plus:
1.4. Make a backup of your complete XAMPP installation (a backup of the folder /Applications/XAMPP).
1.5. Rename the folder of XAMPP into (for example) XAMPP101 (you may need to type a password of your administrator account).
Step 2: Install new XAMPP version
2.1. Place the new XAMPP-installation into the Applications-folder.
2.2. From the backup, put your files back into the htdocs-folder.
2.3. Go to phpMyAdmin and restore your databases one by one. For each database, you need to create one first and after that you will need to import the files you had exported in step 1.2.
2.4 Reconfigure your XAMPP-installation, for example make the nessacery changes in php.ini or httpd.conf. You may also need to add users to MySQL as well if that was needed for your websites. If you didn't had extra users in MySQL in your previous XAMPP-installation, then adding users is not needed.