The Official Radare2 Book

Visual Assembler

You can use Visual Mode to assemble code using A. For example let's replace the push by a jmp:

Notice the preview of the disassembly and arrows:

You need to open the file in writing mode (r2 -w or oo+) in order to patch the file. You can also use the cache mode: e io.cache = true and wc?.

Remember that patching files in debug mode only patch the memory not the file.