To change the tags on a post in Tumblr, you can follow these steps:
1. Open the Tumblr app or visit the Tumblr website and log in to your account.
2. Locate the post for which you want to change the tags. This can be a post on your blog or someone else's blog that you have reblogged.
3. On the post, you'll usually find an icon or text indicating the tags. It may appear as a symbol like a speech bubble or the word "tags."
4. Click on the tags icon or text to access the post's tags.
5. The tags will be displayed in an editable field. You can modify or add tags to the existing ones by typing in the field. Separate each tag with a comma.
6. Once you have made the desired changes, press Enter or click outside the tag field to save the new tags.
Remember, tags help categorize your post and make it more discoverable to others who are searching for similar content on Tumblr. Using relevant and popular tags can increase the visibility of your post within the Tumblr community.