Change function call to another function call in arm using IDA

See this answer for details on how branch instructions are encoded on ARM. Here's what you should do:

1. Set Options->General->Number of opcode bytes to 4 (if it was 0 initially)
2. Find any B instruction.
3. Just to make sure you understand it, work out the displacement from pc+8 at the B instruction to the destination, and verify that the displacement in the instruction matches what you expect.
4. Calculate the displacement to the target to which you wish to branch instead.
5. Use Edit->Patch program->Change byte to modify the current instruction. IDA will update the display, so you can tell whether you've succeeded or not. If you haven't, keep messing with it until you get the bytes you want.
6. To generate a patch file, you can use File->Produce file->Create .DIF file.