I have found an interesting bit of software for modifying 3D files. Its both powerful and intuitive for your average wargamer who really just wants to get the job done so he/she can get back to rolling dice.
SelfCAD (https://www.selfcad.com/) is a Freemium product that you can use to edit your 3D print files make changes, damage things, erase parts, join things together, split things, add pins etc. There is one caveat however, and that that is the Freemium product doesn't do all of the things mentioned above, you have to get the paid version, either on a monthly subscription or a one time purchase.
In the past I have used tinker-cad (https://www.tinkercad.com/) which is great especially for creating things from scratch but as soon as there is texture in the STL files they are too big to be edited in tinker-cad.
Consider the following 3D building, it looks good and prints 'ok' (not well but OK). Once its printed removing from the print bed becomes a major problem. Because the portico has no floor the leverage when removing each of the print bed contact points the leverage on the areas marked with arrows means that its going to snap 95 times out of 100.
The following modified model has had a section of floor duplicated from the centrer of the building, trimmed, and added to the portico joining the columns etc to the main floor therefore removing most of the leverage on the breakable parts and reinforcing the whole section. It prints better as well.
Final note, this is a wargaming blog not a 3D printing blog, if you would rather see less of this content (or more?) let us know in the comments below.