Set symbology’s lower value manually in ArcGIS PRO
Lower class value of the lowest class cannot be manually changed (it was the same in ArcGIS).
You can update by choosing advanced-> sample size-> set sample size up, if you have not chosen “manual interval”. If you have chosen “manual interval”, not even that will work. If you realize too late that not all your data is covered or want to import symbology, there is no way around to set all class values manually again. There is no option only to update the lower value. The new sampling will always change the class values based on the chosen method and the new histogram.
You can actually set the upper value of the first class lower than the automatically estimated lower value. This is different from ArcGIS and changes the situation from being annoying to being a bug.
The new thing is that you can add “out of range” as a new feature in pro, but again the same, there is no way to update the range once you are in manual interval, you can change the upper value for the lowest interval, but the data will be shown as out of range and that is wrong and a bug.
The preferred solution is to be able to set the lower value manually, than you do not need to update with large samples and you can prepare the symbology for possible future lower values in dynamic data. Additionally, it would be easier to move the sampling from the advanced to the histogram-menu.
In the example I imported symbology from another layers based on latitude onto the column longitude. It is completely out of range, but normally I would want to keep the class limits. I calculated the statistics for the lon column in the table and then chose an upper value which definitely lies in the range of Lon. I added out of range symbols, otherwise the map would be simply white. The points would not be selectable. Choosing to update the range during support would change all class limits when anautomatic classification was chosen in the original layer and not update when "maual interval" is chosen.
Yet another instance where ESRI comes up short. QGIS handles this much better. My advice is to create a new category column based on the ranges of the values you wish to symbolize and then symbolize based Unique values. For instance, I want to create 5 classes and one "No Data" class. I write the logic (CASE WHEN is a good option) for the classes and just produce the text outputs in a new column (ddi_cat). 
Note I created the new column in QGIS but there is probably a way to do it in ArcPro. In general, I highly recommend getting familiar with QGIS, especially when it comes to symbology. It has superior UI, functionality and customizability. I generally map things out in QGIS and then transfer them over to ArcPro for the web mapping features.
The lower value is shown and editable in the histogram view as shown in this GIF.
The example @lzk222 shows can also be done with an arcade expression for class breaks or unique value symbology. In that case the expression itself is configuring classes.
@CraigWilliams , I think you missed the point. Users would like to sort by using a 'Lower value' in the graduated symbology classes rather than an upper value.
I just ran into this today and have an example for acres. I would like symbols to go something like this:
≥50
≥ 400
≥ 2000
≥ 4000
≥ 8000
≥ 12000
≥ 20000
≥ 30000
Below is the best that I can do currently:
We could do a lot of modifications to the labels to clean this up but it takes a lot of time and if anything changes, we have to start over again.
Thanks,
Buck
Craigs answer works for me, changing the minimum value in the histogram forces the classes lower than the data minimum values, although I would have never known that was editable, it doesn't look like it is until you double click the number.
On a side note, the classes make it seem like everything below your current class is included since it says the upper value of each class is less than or equal to the value, I think that is misleading if that is actually limited to the set lower value in the histogram.
