First thing you need to do is install and activate the Better Search Replace plugin. Upon activation, you need to visit Tools > Better Search Replace page to use the plugin. Simply add the text string you want to search for and then add the replace text. Then, you need to select the tables to want to look into. The plugin can run a case sensitive search and replace. It can also perform a dry run which allows you to see how many cells are affected by your search without actually replacing the text. If you are satisfied, click on run search/replace button.
If you encounter any troubleshooting, you can check and follow the instructions here.